Physiological Measurements Flashcards
1
Q
What’s the difference between precision and accuracy?
A
Accuracy is how close to the true value something is
Precision is how close together a set of values are
2
Q
What are some factors that should be considered in the development of a new physiological measurement?
A
Patient comfort and safety Environment Interference Access Biological variability Data amount
3
Q
What are the different criteria used in the assessment of a physiological measurement?
A
Technical demands Diagnostic accuracy Diagnostic impact Therapeutic impact Patient outcome Social impact
4
Q
What factors affect the reproducibility of a physiological measurement?
A
Errors
Sensitivity and Specificity
Predictive value
5
Q
What are the main types of errors that should be considered in physiological measurements?
A
Systematic and random measurement errors
Systematic and random physiological erroes
6
Q
What can cause physiological error?
A
Interference or problems with system design can give systematic errors
Fluctuations can cause random errors
7
Q
What can cause measurement error?
A
Operator bias, equipment or technique differences can cause systematic errors
Movement of subject, equipment or environment can give random errors
8
Q
What are the most common reasons for carrying out pulmonary function tests?
A
For patient assessment to look at serial changes, to assess therapy response, to assess compensation claim and as part of pre-surgery assessment
For research purposes to look at epidemiology, growth and development and to investigate disease.
9
Q
What areas can be investigated in pulmonary function tests?
A
Pulmonary blood flow Lung mechanics/ventilation Respiratory control Gas mixing/exchange Other eg ciliary function Pharmacological/metabolic role of the lungs
10
Q
When should patients be referred for pulmonary function testing?
A
In unusual cases of a common problem In particularly sever cases If patient's not responding as expected If pattern of disease has changed If the history taken and physical examination don't match If there's dual/multiple pathology
11
Q
What are the means of airflow obstruction in intrathoracic obstructive disease?
A
Excess mucus secretion
Narrowing of airways due to shortening of airway smooth muscle and/or due to inflammation and oedema of airway lining
Loss of radial traction
12
Q
Why does intrathoracic obstructive disease affect expiration and extrathoracic disease affect inspiration.
A
In an intrathoracic defect, on expiration, airway pressure is less than intrathoracic pressure so area of weakness is narrowed
In an extrathoracic defect, on inspiration, airway pressure is less than atmospheric pressure so area of weakness is narrowed
13
Q
What are the requirements for infant pulmonary function testing?
A
Should be less than 18 months old or older than four.
Needs sedation, medical cover with potential for resuscitation.
Quiet, warm environment with subdued lighting
Infant should be warm, dry and not hungry or thirsty
Should allow plenty of time
Full explanation should be given to parents
14
Q
How does whole body plethysmography work?
A
Works on the basis of Boyle’s law.
Subject is in an almost airtight cabin and breathes through and pneumotachograph. At one point, a shutter is put across the mouthpiece so subject makes an inspiratory effort against the shutter. The alveolar pressure change is measured by the mouthpiece and the change in thoracic volume is measured indirectly by the change in pressure of he cabin.
Thoracic gas volume and airway resistance can then be measured
15
Q
What does Boyle’s law state?
A
Pressure is inversely proportional to volume at a constant temperature.
16
Q
How can airway resistance be calculated from whole body plethysmography?
A
Once thoracic gas volume is calculated, resistance can be calculated using the formula
Resistance = pressure/flow
17
Q
How is thoracic volume measured from whole body plethysmography?
A
Rearranges boyle’s law to give
Thoracic gas volume = (Volume change/Pressure change) x atmospheric pressure
18
Q
What does nitrogen washout measure?
A
Measures dead space.
19
Q
How is a single breath nitrogen washout test carried out?
A
Patient breathes out to residual volume and then takes a maximal inspiration of oxygen. Patient then breathes out slowly and the volume and concentration of nitrogen is monitored.
20
Q
What will the graph of a single breath nitrogen washout look like in someone with uneven ventilation?
A
Phase 3 slope will have an incline.
21
Q
In nitrogen washout, what is the closing volume?
A
The amount of air that remains in the lungs when flow from lower sections of the lungs is greatly reduced of stops altogether. This occurs in expiration as small airways start to close.
22
Q
What pulmonary function tests can be carried out to measure inflammation?
A
Measure exhaled NO which is increased in asthmatics and low/absent in ciliary dyskinesia
Do bronchoalveolar lavage/induced sputum to look for inflammatory cells
23
Q
What should be considered when selecting a suitable study population for calculation of reference ranges?
A
Ethnicity broadly similar to index population.
Age and height range broadly similar to index population
Large numbers of males and females
Consider how ‘normal’ and representative subjects are
24
Q
What should be considered when putting together a methodology for calculation of reference ranges?
A
Ensure equipment is similar throughout
Ensure procedure is similar throughout
Ensure data analysis is consistent
Check for internal consistency at different heights
25
Define sensitivity and specificity in the context of the operational performance of analysis?
Sensitivity - The lowest concentration of an analyte that a test can reliably measure
Specificity - The ability of a test to not falsely cross-react with other substances than that which they are analysing
26
What is an example of a physiological measurement where specificity and cross-reacting is problematic?
Measuring cortisol when patient is being treated with fludrocortisone - gives false positives
27
How are reference ranges calculated?
Normally the mean value of a population +/- 2 standard deviations.
Values in which healthy people will fall in 95 % of the time. 2.5% they will be above the reference range and 2.5% they will be below the reference range.
28
What is standard deviation?
Square root of variance and is used to measure the amount of spread of the distribution of values around the main value
29
What is a measurement?
The gathering of information from the physical world, involving comparison against reference values.
30
What is linearity?
The extent to which data corresponds with the line of identity. If there's non linearity then new values and reference values don't correspond.
31
What is meant by constant bias?
When there's a constant linear relationship with the line of identity but with constant and equal diversion
32
What do physiological measurements do?
Measure the functions and processes of a tissue/organ/system
33
What are some uses of physiological measurements?
```
Diagnosis
Screening
Research
Sports medicine
Patient monitoring
```
34
What is the hierarchy of assessment of physiological measurements?
```
Best is Diagnostic accuracy
(then descending order)
Technical demands = Diagnostic impact
Therapeutic impact
Patient outcome
Social impact
```
35
What is technical demands assessment of physiological measurements?
Looks into whether to test works or not. Involves looking at accuracy, precision, frequency and testing of safety and environment
36
What is diagnostic accuracy assessment of physiological measurements?
Looks at whether the test actually detects disease. Measures sensitivity, specificity, predictive values and ROC curve
37
What is diagnostic impact assessment of physiological measurements?
Assesses whether results from the measurement lead to a diagnosis or not.
38
What is therapeutic impact assessment of physiological measurements?
Assess whether the results from the measurement impact on management of the patient or not
39
What is Patient outcome assessment of physiological measurements?
Looks at whether the results of the measurement help to improve the outcome for patients or not.
Can be assessed by using clinical trials.
40
What is social impact assessment of physiological measurements?
Assess whether the results from the measurement and the consequent actions make a different in terms of life expectancy, quality of life or disability levels
41
What are the effects of measurement error in a physiological measurement?
If it's a systematic error, can affect the accuracy of results. These errors can occur due to the equipment, the technique or the operator.
If it's a random error, can affect the precision of results. These errors can occur due to the equipment, the environment or movement of the subject.
42
What is sensitivity?
How often the test will be positive if a patient has a disease. Also the true positive rate. Calculated by
True positives/(True positives + False negatives) x 100
43
What is specificity?
How often the test will be negative if a patient doesn't have a disease. Also the true negative rate. Calculated by:
True negatives/ (true negatives + False positives) x 100
44
What would the effect be of raising the threshold of a test?
Would mean that there would be more false negatives but little to no false positives. This would reduce sensitivity but increase specificity.
45
How is the positive predictive value calculated?
= True positives/ (true positives +false positives) x 100
46
How is the negative predictive calculated?
= True negatives / (true negatives + false negatives ) x 100
47
What is a ROC curve?
A receiver operating characteristic curve. Plots 1- sensitivity against 1- specificity. The best tests will be in the upper left hand corner
48
What is the function of the middle ear?
Acts as an impedance matching device, so sound isn't all reflected by fluid in the inner ear. Transfers vibrations of tympanic membrane to the oval window, using the ossicles.
49
How does the middle ear act as an impedance matching device?
Tympanic membrane is 20 times the area of the oval window so amplifies pressure from air to fluid.
Malleus and incus act as a lever system to amplify pressure.
50
How is the cochlear duct kept separate from other structures in the inner ear?
Separated from scala vestibuli by reissner's membrane and scale tympani by basilar membrane
51
How are the compositions of auditory endolymph and perilymph different?
Perilymph has low K+ concentration and high Na+ concentration. Endolymph has high K+ concentration and a voltage of +80 mV in comparison to the perilymph
52
What is the endocochlear potential?
+80mV difference between endolymph and perilymph. Is the main driving force for sensory transduction in hair cells.
53
What is the organ of corti?
Lies between central channel and basilar membrane and contains four rows of hair cells which protrude into the endolymph
54
How is sound transmitted from tympanic membrane?
Membrane vibrates, which causes vibration of the ossicles. This decreases the pressure in the scala vestibuli and so alters the position of hair cells. 3 rows of outer hair cells contract to amplify the movement of the basilar membrane. The change in position opens transduction channels, allowing depolarisation and opening of VOCCs. 1 row of inner hair cells then release glutamate which triggers action potentials in afferent neurons, relaying auditory signals to the brainstem.
55
What are some pathological conditions affecting the outer ear and ear canal?
```
Haematoma which can fibrose if it doesn't heal properly, leading to cauliflower ear.
Malformation
Compacted wax
Otitis externa
Trauma
Tumours
Foreign bodies
Tympanic perforation
Atresia
Osteoma
```
56
What are some pathological conditions affecting the middle ear?
Acute and chronic otitis media
Choleasteatoma - keratinised squamous epithelium. Are erosive and expansile so can spread to ossicles and through the base of the skull.
Tympanosclerosis - calcification of tympanic membrane
Otosclerosis - Abnormal growth of bone near the middle ear. Can cause conductive and/or sensorineural loss
Malformation
57
What can cause cochlear dysfunction?
```
Infection
Ototoxicity
Presbyacusis
Noise
Menieres
Anoxia
Congenital
Meningitis - damages organ of corti and so cochlear nerve
```
58
What is an acoustic neuroma?
Vestibular schwannoma. Benign bran tumour of vestibulocochlear nerve
59
What are the differences in sound perception with conductive, sensorineual and recruitment deficit hearing losses?
Conductive involves sound appearing quieter but not distorted and so responds to amplification.
Sensorineural involves sound appearing quieter and distorted so amplification is less effective.
Recruitment deficit is often of cochlear origin, particularly hair cells. Quiet sounds can't be heard and loud sounds are either heard normally or even louder. Patients tend to shout as they can't hear themselves. There's a reduction in the dynamic hearing range
60
What is the purpose of the tuning fork test?
Assesses cognition.
Measures auditory response to a sound generated by a tuning fork and then assesses the gross symmetry of a person's hearing.
Determines whether a conductive deficit is present
Uses basis that sound transfer is normally conducted more efficiently through air than through bone.
61
What is involved in the Weber hearing test?
Tuning fork is placed in the midline of the patient's head and the patient indicates through which ear the sound is loudest, if any.
If the sound is louder on one side then either that sound has conductive loss or the other side has sensorineural loss.
62
What is involved in the Rinne hearing test?
Tuning fork is first held alongside patient's ear and then on their mastoid process and patient is asked to indicate which, if any, gives the loudest sound.
If air gives loudest sound, this is normal and in Rinne positive.
If bone gives loudest sound, this is Conductive loss and is Rinne negative.
63
Evaluate the tuning for test.
Requires minimal equipment and training.
However, doesn't quantify degree of hearing sensitivity, just indicates that there's a problem
is very tester dependent.
Non-test ear must be masked to prevent a false positive test if hearing loss is unilateral.
64
What are the main subjective hearing tests?
```
Tuning fork
Pure tone audiometry
Speech audiometry
Paediatric audiometry
Paediatric visual reinforcement audiometry
```
65
What is involved in the pure tone audiometry test?
Assesses cognition.
Measures auditory threshold to pure tones spanning a fixed range of discrete frequencies.
Sound presented by air or bone conduction and patient responds to the appearance of sound.
Equipment functionality, calibration and test methodology are standardised.
Different pathologies then give different patterns of pure tone audiometry results
66
What are some advantages of pure tone audiometry?
Gives quantifiable measure of sensitivity.
Measure perception and not just function
Results are standardised between different centres
Can differentiate between different causes and pathologies
Results can be used for diagnosis, monitoring and rehabilitation
67
What are some limitations of pure tone audiometry?
Requires high levels of patient cooperation
Hard to get reliable results with the very young, people with learning difficulties or if there's a non-organic deficit
Results are influenced by technique of tester
Needs a quiet test environment
If hearing loss is unilateral then non-test ear needs to be masked.
Results susceptible to learning effects
Pure tone stimuli is of limited relevance to everyday hearing tasks
68
What are some potential sources of error in pure tone audiometry?
```
Environmental or body noise
Tester technique
Concentration or motivation
Tinnitus
Accuracy of transducer placement
Learning effects/habituation/fatigue
Equipment calibration
Test/re-test variability
```
69
What is the purpose of speech audiometry?
Assesses cognition
Measures a person's ability to recognise speech sounds
Pre-recorded word lists are played at calibrated intensities and patient is asked to repeat the words played to them.
Each word consists of 3 phonemes (perceptually distinct units of sound) and the patient is awarded a point for each phoneme repeated successfully.
Range of intensities are used to illicit scores below 10% and up to patient's maximum (ideally 100%)
70
What are some advantages of speech audiometry?
Speech sounds are more physiologically relevant than pure tones
Can help to differentiate between different types of deficits
Can help to identify non-organic deficits
Can help with rehabilitation procedures
71
What are some limitations of speech audiometry?
Material not available in all languages
Results depend on patient cooperation
Not suitable for the very young or people with limited understanding
72
What is the purpose of paediatric audiometry?
Assess cognition
Uses basis that infants show a roughly standard development of hearing pattern.
Assesses auditory function in children over 6 months
Provides a true measure of hearing ability
Allows for intervention if necessary
73
What are the different types of paediatric audiometry?
Distraction testing for children 6-18 months
Cooperation testing for children 18-30 months
Performance testing for children over 30 months
74
What is paediatric visual reinforcement audiology?
Where sounds are linked with visual stimuli to assess the child's audio threshold
75
What are some sources of error for distraction testing?
```
VIsual cueing
Tactile cueing
Auditory cueing eg jewellery
Olfactory cueing eg perfume
Distractor technique may produce over or under stimulation
Rhythmic stimulation
```
76
What does tympanometry assess?
The integrity, pressure and impedance of the tympanic cavity and middle ear.
77
How is tympanometry carried out?
A probe is inserted into the ear canal, surrounded by an airtight seal. Pure tones of varying intensities are then played whilst pressure is applied to the ear drum. Sound will reflect back off the ear drum and this is recorded.
78
How is tympanometry measured?
The reciprocal of stiffness component is expressed as compliance. This is at a maximum when the applied pressure in the external ear canal is equal to the pressure that's within the middle ear cavity.
79
What are some advantages of tympanometry?
Can distinguish between conductive and sensorineural as it detects fluid in the middle ear, any damage to the tympanic membrane, tumours or foreign bodies.
Quick and simple test
Minimal cooperation required except patient can speak, move or swallow
80
What are some disadvantages of tympanometry?
Requires an airtight seal
Extremely high frequency sounds needed for neonates
Doesn't measure hearing can just detect if there's a problem.
81
What is the purpose of otoacoustic emission testing?
Measure the functional integrity of the cochlear outer hair cells on the basilar membrane
82
How are otoacoustic emission tests carried out?
Stimulus sounds are played into the external auditory meatus and all sounds present in the canal are detected, if there's a functional cochlea, there should be a cochlear echo. The frequency content of the emission is then compared with that of the stimulus.
Transient tests stimulate and then record whilst distortion product tests stimulate and record spontaneously
83
What are some advantages of otoacoustic emissions?
Non invasive and quick to administer
Minimal patient cooperation is required
Used for all ages
Reliably indicates the integrity of the peripheral auditory system
84
What are some disadvantages of otoacoustic emissions?
Doesn't measure perception
Very sensitive to outer and middle ear pathologies
Only provides information up to the level of the outer hair cells
Doesn't quantify cochlear sensitivity.
Responses are completely lost with severe hearing losses
85
What does electrocochleography assess?
CN VIII (vestibulocochlear) and electrical potentials originating from there in response to stimulation of the cochlea so also measures functional integrity of the cochlea.
86
How is electrocochleography carried out?
An electrode is placed either extra tympanically or a needle electrode is inserted through the tympanic membrane. Pure tones are played and the electrodes can detect signals and compare them to a skin surface reference measurement.
87
What are some advantages of electrocochleography?
Doesn't require masking of the contralateral ear
Not influenced by sleep, sedation or general anaesthesia
Can be used as intra-operative monitoring of cochlear nerve function
Can determine cochlear sensitivity
Can investigate cochlear pathologies such as menieres disease
88
What are some disadvantages of electrocochleography?
Painful procedure
Doesn't give information for beyond the first segment of the auditory nerve
Doesn't test perception
Greatest signal strength is achieved by the transtympanic method but this is more invasive
Normative data is required for comparison
Cochlear function can't be reliably obtained at less than 1 kHz
89
What do auditory brainstem responses assess?
The brainstem by measuring electrical potentials that originate from the auditory nerve/ brain stem pathway due to cochlear stimulation
90
How are auditory brainstem response tests carried out?
Stimuli is presented by headphones, bone conduction or insert phones and then electrical potentials are measured by scalp electrodes.
91
What are some adantages of auditory brainstem responses?
Very objective
Can diagnose CN VIII or brainstem lesions
Used in newborn screening
Can obtain auditory thresholds in young children and difficult to test subjects
Allows intraoperative monitoring of cochlear and CNVIII function
Not influenced by sleep, arousal, sedation or general anaesthesia
92
What are some disadvantages of auditory brainstem responses?
Small signal size makes test very sensitive to interference
Subject needs to be relaxed or asleep so small children may need sedation or anaesthesia
Doesn't provide information for beyond the brainstem
Interpretation is influenced by conductive and sensory disorders
93
How is the signal extracted from an auditory brainstem response?
Differential amplification - Cancels signals from distant origins
Signal filtering - removes signals with frequencies that are above or below set limits
Artifact rejection - rejects samples with amplitudes outside of set limits
Signal averaging - allows cancellation of signals that aren't timelocked to the stimulus
94
What do cortical evoked responses measure?
Electrical potentials originating from non specific cortical structures, evoked by acoustic stimuli and so can assess the primary cortex.
95
How are cortical evoked responses carried out?
Tone burst stimuli are presented and potentials are measured by scalp electrodes. Provides an objective, frequency specific estimation of sensitivity.
96
What are some advantages of cortical evoked responses?
Can be used in patients that have inconsistent subjective responses
Can be used in patients who can't or won't participate in subjective testing
Can be used for medico-legal patients or those with suspected non-organic losses
Can assess higher auditory function
Can provide an objective estimation of frequency specific, auditory response thresholds.
97
What are some disadvantages of cortical evoked responses?
Doesn't test perception
Shows a large degree of inter- and intra-subject variability
Is affected by the alertness of the patient
Can't be performed in subjects who are asleep, sedated or anaesthetised
Not suitable for paediatrics
Has an extensive testing time
98
What pulmonary function tests can be carried out on children?
From pre-term neonates to 18 month olds, can carry out infant pulmonary function testing where patient is sedated and breathes through an airtight mask. Machine then applies pressure around patient's thorax forcing them to expire. Flow is the measured.
Over four years old, children can undergo spirometry but inbetween these points, children cooperate less and are harder to sedate
99
How can lung volume be assessed?
Helium dilution
Whole body plethysmography
Nitrogen washout
100
What does helium dilution measure?
Functional residual capacity and residual volume
101
What does whole body plethysmography measure?
Thoracic volume and airway resistance
102
What are two applications of measuring exhaled NO?
NO is increased in asthmatics and increases further in exacerbations of asthma
NO is low or absent in people with ciliary dyskinesia
103
What is induced sputum testing?
Patient breathes in a salty vapour. This then loosens any sputum so that it can then be coughed up. Sputum is then tested for cells indicative of inflammation
104
How can the appropriateness of reference values be checked?
Study a group of healthy individuals and see if their data is in line with the prediction
105
What are some physiological reasons for variance in biochemical tests?
```
Age
Circadian rhythm
Menstrual cycle
Food intake
Time after injury
```
106
How can the clinical value of a test be increased?
By using in combination with other tests
By using sequential tests to establish a pattern
By using dynamic tests
By using venous sampling to identify a hotspot
107
What's an example of using venous sampling to identify a hot spot?
For parathyroid hormone. Glands are visualised by ultrasound and then catheters are inserted at each C spine level to identify the area that's producing the most parathyroid hormone
108
What is sensitivity, in relation to chemical pathology?
The lowest concentration of an analyte that can be reliable measured by a test
109
What is specificity, in relation to chemical pathology?
The ability of a test to not falsely cross-react with substances, other than the one that it is claiming to assay. These substances may be closely related chemically.
110
What is biological variation?
Variance in biochemical measurements, due to the physiology of the subject. Normally larger than analytical variation.
111
What is analytical variation?
Variance in biochemical measurements due to the performance of the analysis. Normally smaller than biological variation.,
112
What calculation is done to assess whether a change in chemical pathology results is significant.
The square root of (SDA(squared) + SDB (squared)) has to be more than 2.8 times the value of the standard deviation
113
In what ways can chemical pathology be used for diagnosis?
To make a diagnosis that can't be made without a test
To substantiate a diagnosis made on other grounds
To clarify differential diagnoses
114
In what ways can chemical pathology be used to monitor disease?
In routine biochemistry
Measurement of tumour markers
In hormone assays
For therapeutic drug monitoring
115
In what ways can chemical pathology be used in screening for disease?
Regularly used to screen for congenital hypothyroidism, familial hypercholesterolaemia, phenylketonuria, Down's syndrome and prostate cancer
116
What are some differential diagnoses for an acute abdomen?
```
Pancreatitis
Appendicitis
Perforated bowel
Small intestine volvulus
Ectopic pregnancy
Myocardial infarction
Peptic ulcer disease
```
117
What knowledge is needed to carry out nerve conduction studies?
Knowledge of cellular anatomy
Neuroanatomy
Muscle anatomy
Surface anatomy of nerves
118
What is the aim of nerve conduction studies?
To transmit information from anterior horn cells to muscle and from sensory receptors to the spinal cord, and measure the electrical and chemical changes.
119
What can nerve conduction studies tell us?
Localisation of the problem, ie, site of entrapment, nerves affected, if problem is with muscle, nmj, plexus, nerve root, dorsal root ganglia, anterior horn cell or a mixed peripheral nerve.
Pathophysiology of nerve disorder ie, whether it's senroy, motor or mixed. If it's due to myelination (mostly inflammatory and treatable) or an axonal problem (rarely inflammatory and rarely treatable)
Severity
Disease course
Temporal course
120
What do nerve conduction studies measure?
Conduction velocity. This depends on diameter and myelination
Compound nerve action potential
121
What are the different kinds of compound nerve action potentials that can be gained?
Distal sensory stimulations. Don't normally have much variation in responses so signal in amplified but not dispersed.
Proximal sensory stimulations have wide range of nerve fibres over a longer distance and so signal can be more spread out
Distal motor stimulation involves amplification on summation so shouldn't be much variation (more curvy line)
Proximal motor stimulation shows bigger range of responses so there's decreased amplitude but a wider signal (there's dispersion)
122
How are nerve conduction studies recorded?
Involves placement of electrodes on the skin and a stimulator that sends electrical responses to the nerves. An emg machine then measure the amplitude of the electrical response.
123
Where are electrodes placed in a motor nerve conduction study?
Recording electrode over the centre of the muscle belly
Stimulate over the nerve that supplies the muscle
Reference electrode over the distal tendon.
124
What currents are used for a motor nerve conduction study?
Starts at 0mA and is then increased slowly to around 20-30mA until maximal stimulation is reached, whereby all nerve fibres are excited.
125
What is measured in a motor nerve conduction study?
Velocity
Latency (ms) - shows time taken from stimulus site to the NMJ, time delay across the NMJ and the depolarisation time across the muscle.
Amplitude - Reflects the number of muscle fibres that fire and therefore the number of active axons. The close the recording electrode to the source, the higher the amplitude.
Duration - measure of fibre synchrony. It's increased if some muscle fibres are slowed.
126
What may affect latency in a nerve conduction study?
Affected my demyelination
127
What can affect the amplitude in a motor nerve conduction study?
Affected by the loss of axons or by conduction block. Therefore may be altered in NMJ disorders and myopathies.
128
What's involved in a sensory nerve conduction study?
Peripheral nerve is electrically stimulated and then recording is taken over a purely sensory portion of that nerve's distribution.
129
What currents are used for sensory nerve conduction study?
Starts at 0mA and is increased slowly with stimulations lasting 200microseconds. Current is increased to 5-30mA depending on when supramaximal stimulation is reached, whereby further increase no longer affects response. In general, lower current is required than in motor nerve conduction studies
130
How is velocity calculated with 1 and more than 1 stimulation sites?
1 stimulation site = (distance between recording and stimulating electrodes)/onset latency
more than 1 stimulation site = distance between stimulation sites/(latency proximal - latency distal)
131
What is measured in a sensory nerve conduction study?
Latency - time from stimulus to Sensory Nerve Action Potential onset/peak
Amplitude - Reflects number of sensory fibres depolarising. Proportional to proximity of recording electrode to nerve
Duration - measure of synchrony. Normally less than Compound Nerve Action Potential
Velocity
132
What are some problems and artefacts that occur in nerve conduction studies?
```
Patient discomfort
Patient factors
Wrong question asked
Equipment failure
Electrical interference
Overcalling clinical abnormalities without clinical correlation
```
133
What causes patient discomfort in nerve conduction studies?
An unpleasant or painful stimulation. If this is particularly problematic, sub-optimal stimulation may be used and this risks artifactually low readings
134
What are some patient factors that can affect nerve conduction studies?
Movement of subject causes noise in signal
If limbs are too cold, causes artifacts. May need to be warmed gently.
Peripheral oedema increases the distance from the recording electrode to the nerve/muscle
Inaccessible areas, eg due to bandages
Syncope
135
Why may abnormalities be over-called in nerve conduction studies?
Is a very sensitive test with a wide range of normal values. Mild, subclinical changes may not be of significance so test needs a clinical correlation
136
What are the main clinical indicators for carrying out nerve conduction studies?
To investigate paraesthesias and limb weakness.
| To help diagnose carpal tunnel syndrome, guillain-Barre syndrome, peripheral neuropathy and spinal disc herniation
137
What are the main pathological alterations seen in a nerve conduction study?
axonal loss - much reduced signal amplitude
Uniform demyelination (inherited) Some delay in latency
Non-uniform demyelination (acquired) Near complete loss of signal
138
What's the purpose of eegs?
Record electrical activity in the brain by measuring the sum of many neurons with the same spatial orientation.
139
How do EEGs interpret signals from neurons?
Neuronal networks have intrinsic rhythmicity that can be detected on EEG. The EEG recording can then be broken down into frequency contributions and spatial distribution.
140
How is electrical activity measured in an EEG?
Electrodes are placed over the scalp (dural to localise epileptic activity) These electrodes are placed in set montages over the scalp bilaterally. Electrodes are in pairs with an amplifier for each pair. Recordings are then represented as montages.
141
Where are electrodes placed in an EEG?
```
Generally:
Frontal
frontopolar
temporal
occipital
central
parietal
auricular.
Any in the midline are suffixed Z and any on the right have an even number attached and any on the left side have an odd number attached ie, Fz for frontal central. A1 from auricular on the left.
```
142
What are the three main types of montages in an EEG?
Bipolar - each channel represents difference in voltage between two adjacent channels.
Referential - each channel represents difference in voltage between an electrode and a reference electrode, often in the midline.
Average reference - outputs of all amplifiers are summed and averaged. The average signal is then used as the common reference for each channel.
143
What are the four main rhythms seen in an EEG?
Delta (less than/equal to 4Hz)
Theta (4-8 Hz)
Alpha (8-13 Hz)
Beta (more than 13 Hz)
144
When is an alpha signal physiological on an EEG?
In a normal person in wakefullness but with eyes closed, measured over the back of the head
145
Which signal is particularly abnormal to see in wakefullness on an eeg?
Delta
146
What can an EEG show?
```
Frequency
Amplitude
Quantity
Morphology
Reactivity
Variability
Topography
Phase relationships.
```
147
What kind of activation procedures are used in an EEG?
Hyperventilation
| Photic stimulation
148
In an EEG, what do visual evoked potentials assess?
The pathway between stimulus ie from retina, and the visual areas of the brain ie occipital lobe.
149
In an EEG, what do somatosensory evoked potentials assess?
The pathways between sensory nerves and the sensory parts of the brain
150
What are the main clinical indicators for carrying out an EEG?
Identification of specific epilepsy syndromes, which could impact on a patient's prognosis and treatment
Localisation of a specific area of the brain where a seizure starts, which can have implications for surgery
Distinguishing between epilepsy and other causes
To rule out/ distinguish some neurological and psychiatric conditions.
151
What specific form of epilepsy can an EEG diagnose?
Primary generalised epilepsy in children showing a 3Hz spike and wave pattern
152
What are some advantages of EEG?
Cheap
Excellent temporal resolution
Can diagnose some specific epilepsy syndromes
153
What are some disadvantages of EEG?
```
Poor spatial resolution
Poor detection of deep brain activity
Needs training and experience to perform and interpret
Changes may be physiological
Requires patient cooperation
```
154
What are some artifacts found in EEGs?
Biological eg due to blinking/eye movement
Environmental eg due to electrical interference. Can have huge effect as EEG signals are greatly amplified
Technical - lead misplacement or disconnection
155
What do EMGs do?
Measure the electrical activity of skeletal muscle. Electrical activity comes from the muscle membrane
156
What is a motor unit?
A single motor neuron and all of the muscle fibres that it innervates.
157
When may surface electrodes be used for an EMG?
To give a general picture of muscle activation ie in a sleep study
158
How are electrodes used in EMGs?
Needle electrodes. Needle inserted that inserts the electrode into the muscle
159
What can EMGs be affected by?
Motor unit composition
Number of muscle fibres per motor unit
Muscle fibre abnormalities
Spontaneous activity of denervated muscles.
160
What artifact can be produced by needle insertion in EMGs?
Causes electrical activity as insertion damages muscle fibres.
161
How can you tell if the needle has landed on an end plate in an EMG?
Muscle is electrically silent at rest whereas end plates tend to be very spontanteously active. If muscle is at rest but there's electrical activity then the electrode needs repositioning.
162
What is produced by voluntary contraction on an EMG?
An interference pattern
163
How does nerve damage appear on an EMG and why?
The controlling neuron of a motor unit is lost and some muscle fibres released from that unit can fire spontaneously, giving distinctive fibrillation and positive sharp waves.
Released muscle fibres can be taken up by other motor units causing abnormally large motor units and therefore abnormally large action potentials. However, the overall number of motor units is decreased so the interference pattern is less full at contraction and there's increased duration of action potential
164
How long can denervated muscle fibres remain viable?
Around 7-10 days
165
How does muscle damage appear on an EMG and why?
There's a reduced duration of the action potential and a reduced area for amplitude. If it's very severe, can be reduction in the number of motor units. Axons are firing but can't exert a full effect
166
What technical artifacts can occur in an EMG?
Electrical interference
Inaccurate needle placement
Skeletal muscle is homogenous so if too few areas of the same muscle are sampled then can give a misleading impression.
167
What clinical problems are there with EMGs?
Unpleasant and invasive
| Risk of haematoma
168
What clinical artifacts can occur in EMGs?
Wrong question asked
In very longstanding, wasted muscle, fibres can become replaced with fat and fibrous tissue. This is more difficult to measure from and less informative.
169
What are the 3 main sleep/wake states for humans and other mammals?
Wakefulness - cortex is active. Memories are stored and motor system is capable.
Slow wave/non-REM - Cortex is relatively inactive but motor system is capable. Memories aren't stored.
REM sleep - cortex is active but motor system is inhibited. Memories aren't stored.
170
What are the main electrophysiological characteristics of slow wave sleep?
```
Low frequency, high amplitude
Synchronised
4 recognised stages
Decreasing frequency and increasing amplitude with increasing depth
Slow rolling eye movements
Motor system capable
```
171
What are the 4 stages of slow wave sleep?
Stage 1 - very light sleep
Stage 2 - light true sleep. EEG shows spindles and K complexes
Stage 3 - deeper sleep
Stage 4 - deeper sleep
172
What are the electrophysiological characteristics of REM sleep?
```
High frequency, low amplitude
Desynchronised
Ill defined depth
Rapid eyemovements
Skeletal muscle paralysis, except for respiratory muscles
```
173
What are the main classes of sleep disorders?
Disorders of excessive sleepiness
Difficulty in initiating and maintaining sleep
Circadian rhythm disorders
174
How can disorders of excessive sleepiness be further subclassified?
As primary. due to a primary brain problem of sleep fragmentation
Or secondary due to broken sleep from multiple arousals
175
What is narcolepsy?
Breakdown of the barriers between wakefulness and sleep, allowing inappropriate transitions
176
What are the 5 core symptoms of narcolepsy?
```
Hypnogogic and hypnocampic hallucinations
Excessive daytime sleepiness
Cataplexy
Sleep paralysis
Disturbed night time sleep.
```
177
How can excessive daytime sleepiness manifest?
As continuous day time sleepiness or as irresistible sleep attacks
178
What is cataplexy?
A bilateral, sudden loss of muscle tone with preserved consciousness. Can be partial or complete and the knees, face and neck are most commonly affected. Normally provoked by emotion or anticipation of emotion and a full attack takes a few seconds to develop.
179
What are some associated symptoms of cataplexy?
Breathing difficulties
Sweating
Palpitations
180
How does cataplexy present?
```
Arm or leg weakness
Sagging jaw
Nodding head
Slurred speech
Full cataplexy attack in complete atonia and postural collapse
```
181
What are hallucinations?
Vivid dream-like experiences that occur during the transition between wakefulness and sleep
182
What's the difference between hypnogogic and hypnopompic hallucinations?
Hypnogogic occur in transition from awake to sleep.
| Hypnopompic occur in transition from sleep to wakefulness.
183
What is sleep paralysis?
A brief inability to move with preservation of consciousness. Occurs on sleep or waking and can be intensified with hallucinations. Ends spontaneously or after mild sensory stimulation
184
What brain pathways promote sleep?
Ventrolateral preoptic nucleus releases inhibitory GABA and galanin that antagonises brainstem components that promote wakefulness. VLPO also inhibits hypocretin which strengthens arousal centres
185
How is hypocretin affected in narcolepsy?
Transmission of hypocretin is lost
186
What are the primary disorders of excessive sleepiness?
narcolepsy
Long sleepers
Cerebral injury
Idiopathic hypersomnolence
187
What are the typical signs and symptoms of obstructive sleep apnoea?
```
Excessive daytime sleepiness
Unrefreshing sleep
Snoring
Witnessed apnoeas
Obesity
Large neck
Crowded oropharynx
Comorbidities
```
188
What are some common comorbidities with obstructive sleep apnoea?
Type 2 diabetes mellitus
Ischaemic heart disease
Hypertension
189
What are some consequences associated with obstructive sleep apnoea?
```
Reduced quality of life
Reduced cognition
Mood changes
Excessive daytime sleepiness
Nocturia
Accidents
Impaired glucose intolerance
Hypertension
Cardiovascular disease
```
190
What is apnoea?
Cessation of airflow for over 10 seconds
191
What is hypopnoea?
A 50% decrease in airflow for over 10 seconds
192
What is central apnoea?
Reduction in airflow due to decreased effort.
193
What is obstructive apnoea?
Lack of airflow due to obstruction
194
What are some anatomical features that influence obstructive sleep apnoea?
```
Overbite
Large tongue
Narrow maxillary arch
Long soft palate/uvula
Large tonsils
Poor nasal airway
```
195
What are some secondary disorders of excessive sleepiness?
Obstructive sleep apnoea
Central sleep apnoea
Limb movement disorders
196
What are some disorders of difficulty initiating and maintaining sleep?
```
Insomnia
Depression
Poor sleep hygiene
Drug induces
Fatal familial insomnia
Paradoxical insomnia
```
197
What is sleep hygiene?
The behaviours and environments surrounding sleep.
198
What are some circadian rhythm disorders?
```
Delayed sleep phase syndrome
Advanced sleep phase syndrome
Jet lag
Long sleepers
Non-entrained circadian rhythm
```
199
In what group of people in a non-entrained circadian rhythm most common?
People who are blind from birth with non-functioning retinas
200
What is insomnia?
An inability to initiate and maintain sleep
201
What is hypersomnia?
An inability to stay awake
202
What is parasomnia?
A category of sleep disorders where sleep is disturbed by movement, behaviours, emotions, perceptions and dreams
203
What are some acute effects of sleep loss?
```
Decreased performance and alertness
Reduced memory and cognition
Decreased interpersonal communication
Reduced quality of life
Increased risk of accidents
```
204
What are some effects of chronic sleep loss?
Increased risk of hypertension, MI, heart failure, stroke, obesity, psychiatric problems, ADHD
Mental impairment
Risk of foetal and childhood growth retardation
205
How can sleep apnoea be assessed?
```
Overnight pulse oximetry
Home respiratory sleep study
Polysomnography
Apnoea hypopnoea index
Diagnosis criteria
```
206
What is measured in a home respiratory sleep study?
SpO2
Airflow
Thoracic and abdominal effort
Maybe ECG
207
How can a home respiratory sleep study differentiate between central and obstructive sleep apnoea?
Using thoracic and abdominal effort.
If this reduces followed by reduced airflow, is central.
If there's increased effort associated with reduced airflow, is OSA
208
What is actigraphy?
Assesses limb movement in sleep, normally at the wrist. If it detects movement, subject is awake. Good for assessing sleep pattern.
209
What is polysomnography and what does it involve?
```
A comprehensive inpatient sleep study.
Investigates:
EEG
ECG
EMG
EOG
heart rate
chest wall movement
Airflow
Airway pressures
Oximetry
COntinuous positive airway pressures
Body movement
VIdeo
```
210
What is the Apopnoea Hypopnoea Index?
Measures frequency of apnoea/hypopnoea events per hour.
211
Using the AHI index, how is obstructive sleep apnoea classified?
Mild if 5-14 events per hour
Moderate if 15-29 events per hour
Sever if more than 30 events per hour
212
What are the diagnostic criteria for obstructive sleep apnoea?
Has to be over 4% drop in oxygen saturation
Also has to be A+B+D or C+D
A - more than one of: symptoms of excessive daytime sleepiness, fatigue or insomnia; waking with apnoea/gasp/choke; partner reports loud snoring/apnoea
B - PSG evidence with obstructive AHI more than/equal to 5
C - PSG evidence with obstructive AHI more than/equal to 15
D - Not better explained by another sleep disorder, medical disorder, drug or substance abuse
213
How can daytime sleepiness be assessed?
Subjectively using stamford/Epsworth sleepiness scale
| Objectively using multiple sleep latency test, vigilance test or assessment of wakefulness test
214
What is the stamford sleepiness scale?
Uses a situation scale of tiredness ie alert/a little foggy/sleepy etc for certain situationa
215
What is the epsworth sleepiness scale?
A self administered questionnaire where patients rank 0-3 over of likely they would be to fall asleep in 8 given situations such as reading, in traffic etc.
If 0-10, normal
216
What is the multiple sleep latency test?
Measures the latency in 4-6 15-20 minute naps placed 2 hours apart. Subject placed in a dark room and told to try and fall asleep. Minutes to sleep onset and then minutes to REM sleep onset are then measured.
If mean sleep latency is less than 5-8 and there's more than/equal to 2 episodes of REM sleep then this is diagnostic of narcolepsy
217
What is the assessment of wakefulness test?
Patient given 4 nap oppurtunities of roughly 20 minutes and told to try and stay awake. If mean sleep latency is more that 11 minutes then this is normal.
Good for assessing unintended sleepiness
218
What is the vigilance test?
Subject is placed in a dark room 4 times, each of 40 minutes length. A light is presented every 3 seconds and the subject must press a button in response. If there's no response for more than 21 seconds, patient is said to be asleep.
219
What kind of vascular pathological conditions can be investigated using ultrasound?
```
Major risk factors for stroke ie carotid artery atheroma
Narrowing of arteries
Abdominal aortic aneurysms
Peripheral vascular disease
Severe vessel calcification
```
220
What is the doppler effect?
Describes how an object moving towards the observer will emit a sound with a higher frequency than an object that's moving away from the observer
221
What can doppler ultrasound reveal?
Narrowing of arteries
Blood flow in a foetus
Deep vein thrombosis
222
How does non-imaging doppler provide insight to blood flow?
Emits ultra high frequency sound waves via a piezoelectric crystal. These sound waves are reflected by the blood cells passing beneath it and this reflection is detected by another crystal. This produces a pulsating sound that can be amplified and made audible via speakers
223
What equation can be used to determine velocity from a doppler ultrasound?
Doppler shift = ((2.Velocity.Frequency) . Costheta) / constant
If doppler shift is known, can determine velocity of flow.
224
Why shouldn't a doppler transducer be placed at a right angle?
As equation for doppler shift involves multiplying by Costheta. Cos(90) equals 0 so if transducer is at a right angle, there will be no doppler shift
225
What is a duplex scanner?
Ultrasound that can do two things. Scans in brightness and colour flow mode.
226
What is the B mode on ultrasound?
Transducer contains many piezoelectric crystals to scan a plane through the body.
Different tissues reflect sound waves in different ways depending on the echogenicity. Pure fluid absorbs all sound, solids reflect sound and gas reflects all sound.
Pure fluid therefore, is anechoic so any solid distal to it will appear brighter as all the sound is absorbed by the fluid and then reflected by the solid.
227
What is acoustic enhancement?
The process whereby solid distal to fluid on ultrasound appears brighter than surrounding tissues
228
What is acoustic shadowing?
The process whereby soft tissues distal to gas can't be visualised so is anechoic as all sound is scattered by the gas
229
What does colour flow imaging do on ultrasound?
Superimposes a colour coded map onto an ultrasound image. Converts doppler shift and frequency into colour depending on flow. Can also show velocity with further shading of colour shown
230
What is colour flow imaging particularly good for?
Exhibiting turbulent flow through a vessel, eg in stenosis
231
What is spectral imaging?
Also called pulsed wave doppler. Operator selects a specific zone and then velocity of this particular area is shown. Computer then shows a pulse line
232
How can duplex scanning investigate abdominal aortic aneurysms?
B mode can show the dilated aorta and colour flow imaging can then show the flow through the dilated vessel
233
How can duplex scanning investigate peripheral vascular disease?
PVD can be picked up with non-imaging doppler but duplex can then show narrowing of artery, turbulent flow and a zone of increased velocity with an altered wave form.
234
What signs indicate deep vein thrombosis by investigation with duplex scanning?
```
Visible thrombus
Non-compressibility
Failure of venous distension on valsalva
Lack of normal venous Doppler signal
Loss of flow on colour images
```
235
How can duplex scanning investigate venous insufficiency?
Ultrasound used alongside compressing and then releasing calf. If reflux lasts for longer than 0.5 sec, there's venous insufficiency
236
How can abdominal aortic aneurysms be monitored using ultrasound?
Men over 65 are screened for AAA.
If aorta is less than 3cm, doesn't need monitoring.
If aorta is between 3-4.4 cm, should be monitored yearly
If it's between 4.5-5.4 should be monitored monthly
If it's over 5.5, should be referred for treatment.
If it increases in size by more than 1 cm in a year, should be referred for treatment
237
How is non-imaging doppler used to investigate peripheral vascular disease?
Transducer is placed over a foot pulse point and then blood pressure is taken using a cuff around the ankle, much like a normal brachial pressure but only systolic pressure is taken. Brachial blood pressure is then also taken. Then calculate the ankle brachial pulse index =
Ankle systolic pressure/brachial systolic pressure
238
How is ankle brachial pulse index graded?
```
If >1.4 = incompressiblity due to calcification
1 - 1.4 = probably no disease
0.81 - 1 = mild/insignificant disease
0.5 - 0.8 - moderate disease
<0.3 = critical ischaemia
```
239
How are carotid stenoses graded using information for ultrasound?
Use internal carotid peak systolic velocity in cm/s
If 400 = >90%
If variable = near occlusion
If no flow = occlusion
240
What are some advantages of duplex scanning?
```
Provides haemodynamic information
Gives a 'live' image
Non invasive
Cost effective
Highly sensitive and specific
No known side effects
Transportable
```
241
What are some disadvantages of duplex scanning?
Highly operator dependent
| Can't penetrate gas or bone so pancreas and lungs are very difficult to visualise.
242
What does the spatial detail of ultrasound depend on?
Frequency.
High frequency will give a higher resolution but a lower depth penetration
Low frequency will give lower resolution but better depth penetration
243
What is nuclear medicine?
The use of very small amounts of radioactive material which is incorporated into compounds or pharmaceuticals. These are then distributed throughout the body so that different structures and tissues can be visualised
244
What is scintigraphy?
The use of plain photographic film which gets exposed by radioactive compounds taken up into the body. This can then give a 2D image of a certain structure
245
What is the most common use of scintigraphy?
Using Iodine^123 which accumulates in the thyroid. This can then assess hyperthyroidism/hyperparathyroidism
246
What is single photon emission computed tomography?
Using a gamma ray emitting radiopharmaceutical. This is taken up by certain structures, particularly the heart. Many slices of images are then taken to give a 3D picture
247
What is positron emission tomography?
Use of a beta particle emitting radiopharmaceutical, most commonly fluoro-deoxy-glucose. The beta particle will decay to emit a positron and an electron and this emission will be visualised. Fluoro-deoxy-glucose has an almost identical structure to glucose so can be metabolised by cells. Therefore, more quickly dividing and highly metabolic cells will take up more than other tissues so these metabolic 'hot spots' can be visualised so is used to detect cancers.
248
What is optical imaging used for?
Uses optical polarised light which can give good resolution and real colour of structures.
Also optical coherance tomography reflects polarised light into the eye to visualise retinal layers
249
How are x rays produced?
High speed electrons are bombarded towards a metal target. The high speed is generated by the difference in charge between the beginning cathode and ending anode. X ray photons are then emitted and reflect off the metal target (usually tungsten) and can then pass through the subject and will expose photographic film placed on the other side of the subject.
250
How are different tissues visualised on x ray?
The way that a tissue appears on X ray film depends on that tissue's attenuation of the x ray beam. Bone has high calcium which absorbs the X rays well. However, the air content in lungs has poor absorption and so lungs can be seen in contrast to the other tissues around it.
X rays are good for contrast between tissue but not as good for differences within tissue so that bone, lung and abdominal disease can be easily visualised but brain and muscle pathologies cannot be detected.
251
How is computer tomography carried out?
Patient is put through a tube that has an x ray source emitting beams through the patient, with detectors on the other side. These images are taken in many different planes in order to get a 2D reconstruction
252
What are the benefits of computed tomography, compared to other radiological imaging techniques?
Provides a greater detail than x ray and has much better spatial resolution than MRI.
Is very quick.
Good for internal organs and blood vessels
253
What are some disadvantages of CT compared to other radiological imaging techniques?
Higher radiation dose
| Not very good at contrast within soft tissue, such as brain and muscle.
254
What is meant by a paramagnetic nuclei?
One that is only magnetised when they have a magnetic field applied to them
255
What is the basis of magnetic resonance imaging?
That all atoms consist of a nucleus, surrounded by electrons. An atom's nucleus consists of protons and neutrons which have 'spin'. A spinning charge gives a magnetic field.
Also that hydrogen is a paramagnetic atom.
256
How are images produced by magnetic resonance imaging?
The patient is magnetised with an extremely powerful magnet (1.5-3 tesla) and the higher the tesla rating, the clearer the picture.
The magnet makes all the hydrogen nuclei face the same way however some nuclei will be unmatched. A radiofrequency is then applied which spins any unmatched nuclei so that they're facing the right direction. When the radiofrequency is turned off, unmatched nuclei return to their original position and this spin emits energy which can be picked up by gradient coils within the machine which send the information to a computer which can analyse the information and transform it into a picture.
257
What is the larmour frequency?
The frequency at which the nuclei in an MRI are spun at by the radiofrequency pulse. The frequency is proportional to the strength of the magnetic field.
258
What is Bayes' theorem used for?
A way to update probabilities, based on new evidence
259
How is odds ratio calculated, using probability?
= P(hypothesis is true - H)/ (1-P(H))
260
Using odds ratio, how can the probability of H being true be calculated?
P(H) = OR/(1+OR)
261
How is likelihood ratio calculated?
P(Evidence is true, given H is true) / P (E is true, given H isn't true)
262
How is a posterior odds ratio calculated?
Prior odds ratio x likelihood ratio
| P(H|E)/(1-P(H|E)) = P(H)/(1-P(H)) x P(E|H)/P(E|H')
263
How can bayes' theorem be applied when we have multiple pieces of evidence?
If E1 = previous evidence and E2 = new evidence
| P(H|E2,E1)/(1-P(H|E2,E1)) = P(H|E1)/(1-P(H|E1) x P(E2|H)/P(E2|H')
264
What's the configuration of receptors/beams in a third generation CT scanner?
Linear array of many detectors, opposite the x-ray beam which emits a beam in a fan shape. The detectors and x ray rotate together, rather than rotate-translate.
Also now spiral so there's continuous rotation of the tube and patient is continuously and slowly and smoothly moving through the tube
265
What does the image intensity of a ct scan depend on?
The hounsfield unit of the the tissue (HU) air has HU of -1000 and metal has hounsfield unit of +1000 and HU of water = 0 and they're all depicted as different shades of grey
266
How do hounsfield units relate to the colour of a tissue on a CT?
Relates to the width and centre of the image. THe 'width/window' is the range of CT numbers/HU that are displayed on the greyscale and the centre determines the number around which the width is created.
A negative width means these colours will appear black and a positive width appears white
267
What are the different methods of giving contrast in a ct?
Intravenously
rectally
orally
268
Why is intravenous contrast used?
To help to highlight vasculature as well as come soft tissue organs, ie liver and kidneys as well as the spine.
269
How does contrast medium help to make sharper images on CT?
Contrast is slightly radioactive. When the CT x-ray beam passes through the contrast, the beam is attenuated/weakened and so the area with the contrast in it will appear whiter and therefore more different than surrounding tissues so structures can be better visualised
270
Why is oral contrast used in CT?
Predominantly to help visualise structures of the abdomen and pelvis
271
Why is rectal contrast used in CT?
To visualise large intestines and other organs in the pelvis
272
Why is the timing of oral contrast important in CT?
Needs proper preparation and timing so as to be able to reach required organs. Given in roughly four doses over 2-3 hours to give it time to reach bowel and bladder with a final dose given just prior to the CT so that the stomach and duodenum can be visualised
273
Why is the timing of intravenous important in CT?
Done immediately before scan. Needs roughly 30 seconds to be injected and to distribute around the body. Highly vascularised structures will have a higher supply and so will appear more highly contrasted
274
How can ulcerative colitis and crohn's be differentiated between on CT?
Ulcerative collitis will give a 'collar button' ulcers appearance as there's ulceration through the mucosa into the muscle and then up and down in a 't' shape
Crohn's will show a 'string sign of kantor' on CT due to areas of narrowing or stricturing, surrounded by areas of dilatation.
275
How can contrast timing affect diagnosis on CT?
Tissues will absorb contrast differently and timing of contrast in comparison to imaging could mean that some areas are less visible than they otherwise might be. This means that certain diagnoses could be missed.
Malignant lesions also show different contrast enhancement patterns over time.
276
How can colonic transit time be studied?
By undergoing a SHAPES test.
A sitzmark capsule is swallowed on day 0 and patient told to refrain from laxatives. The capsule contains numerous markers that show up on x-ray. A plain AXR is then taken on day 5 to visualise the radioopaque markers and see their distribution. If at least 80% have been expelled, transit time is normal. If >6 have been retained, need follow up x-ray in a few days. If markers accumulate in rectosigmoid or diffusely, need to proceed onto 2nd step.
Take a bulking agent and encourage fluid intake. Take another capsule with another x-ray in 5 days.
If >80% have been eliminated - normal.
If markers are scattered diffusely - most likely hypomotility or colonic inertia
If markers have accumulated in rectosigmoid - most likely has a functional outlet obstruction eg rectal prolapse
277
Why have CT scans overtaken urography for investigating renal colic?
Ct can show renal stones and hydronephrosis without the use of contrast that was required in urography. Less damage to kidneys.
278
How can PET/CT be used to stage cancer?
Cancer staging is process of describing the size of a cancer and the way that it has grown. Most often looks at tumour size and spread to any other parts of the body. Both of these can be visualised on CT and PET and so staging can be done on the basis of these scans
