Imaging Techniques Flashcards
1
Q
What are X-rays ?
A
- Are a form of HIGH ENERGY RADIATION belonging to the electro magnetic spectrum, 10-10 lambda(m)
- They pass thru the body undergoing Differential ABSORPTION by tissues
2
Q
How do X-rays in radiography work?
A
- X-rays fall onto fluorescent film- screen combinations -> black silver crystals ( film blackening) these processed to give an image.
- The tissues containing high atomic no- e.g. calcium in bone, absorb a higher amount of X-rays (high beam attenuation) -> resulting in less crystals formation on the film & therefore a white radiodense appearance.
- Tissues where less absorption - e.g. fat- ( low beam attenuation- give black radiolucent appearance as more crystals are formed on the film. Muscles appear grey as intermediate attenuation
3
Q
How are X-rays produced?
A
- Heating a fine filament usually made of Tungsten ( negative cathode) to INCANDESCENCE in a VACUUM to around 220oC-> Emission of ELECTRONS ( thermionic emission)
- Electrons leave cathode move to positive anode- a smooth metal fragment of tungsten.
-
Electrons hit tungsten at about half speed of light on the FOCAL SPOT where they either interact with
- 1) outer electrons of target nucleus -> heat
- 2) inner electrons- knocking them off orbit-> X-RAYS
- 3) nucleus - >electrons slow down! change direction -> X-rays from BRAKING radiation ( 80% production of xray made this way)
- As X-rays pass thru pt they can be Transmitted, absorbed, scattered -> attenuation After passing thru pt fall onto X-ray plate
4
Q
What is the X-ray cassette made up of?
A
- Carbon fibre/ aluminium front - filter to minimise beam attenuation , remove low energy X-rays , reduce pt exposure
- Lead sheet back- decrease backscatter of radiation
- Film- polyester based coated with fine photographic emulsion - silver iodiobromide- sensitive to X-rays
- Two intensifier screens on each side of the film-polyester base coated with dense layer of phosphor crystals that absorb X-rays and converts them into visible light which exposes the film
5
Q
What is involved with processing of the film?
A
- Development by alkaline immersion
- Fixative in acid immersion
- Wash Dry
6
Q
What is the difference with digital imaging?
A
- Digital Xrays is a form of X-ray imaging, where digital X-ray sensors are used instead of traditional photographic film.
- Indirect FPDs. Amorphous silicon (a-Si) is the most common material of commercial FPDs.
- Combining a-Si detectors with a scintillator in the detector’s outer layer, which is made from caesium iodide (CsI) or gadolinium oxysulfide (Gd2O2S), converts X-rays to light.
- Because of this conversion the a-Si detector is considered an indirect imaging device.
- The light is channeled through the a-Si photodiode layer where it is converted to a digital output signal.
- The digital signal is then read out by thin film transistors (TFTs) or fiber-coupled CCDs. The image data file is sent to a computer for display.
7
Q
What are the advantages of digital radiography?
A
- Advantages include time efficiency through bypassing chemical processing
- ability to digitally transfer and enhance images.
- Also less radiation can be used to produce an image of similar contrast to conventional radiography.
8
Q
What are the advantages of X-ray radiology?
A
- Good for assessing bone
- Cheap
- Easily obtained
9
Q
What are the disadvantages of X-ray radiology ?
A
- Not sensitive to subtle bony destruction or abnormalities
- Difficult to interpret in areas of complex anatomy
- Form of ionising radiation -aim to reduce dose
10
Q
What does Pacs stand for ?
A
- Picture archiving and comminution systems
11
Q
What are the disadvantages of digital X-rays ?
A
- Do not have the high spatial resolution of radiographic film- screen combinations
- Expensive though costs reducting
12
Q
What is ultrasound ?
A
- An imaging modality which used HIGH frequency (3-50Mhz) SOUND Waves generated by a PIZOELECTRIC crystal transducer.
- The waves are reflected and refracted at the tissue interface
- Reflected waves return to the Transducer where they are converted into electrical signals in order to -> image
13
Q
How does ultrasound work?
A
- Sound waves produced from PIZOELECTRIC CRYSTAL.
- A DC VOLTAGE is applied & then reversed across the crystal which expands/ contracts- changes shape -> compression sound waves
- When Coupled to the skin using lubricating gel sound waves are transmitted into the body and reflected back.
- When sound waves arrive back at the transducer-> distort shape of PIZOELECTRIC crystal -> voltage
- The more sound waves reflected the greater the voltage -> brighter the image
14
Q
How does higher frequency of the wave effect the overall image?
A
- The higher the frequency the higher spatial resolution , the attenuation within the tissue is higher- so limiting visualisation to specific structures
15
Q
What is uss used for?
A
- Assessing tendons
- Assessing masses
- cystic= hypoechoic appearance
- solid= hyperechoic appearance
- Confirming joint effusions
- with guidance of injections
- Screening and evaluation of DDH
16
Q
What are the advantages?
A
- NO IONISING RADIATION
- No side effects
- Painfree
- Non invasive
- Machines
- SMALL
- ** INEXPENSIVE**
- PORTABLE DOPPLER added to observe Vascularity and blood supply- dynamic
17
Q
What are its disadvantages?
A
- High operator dependance for acquision and intrepretation
- Field of view is often limited
- Difficult to characterise the imaged tissue
- Cannot penetrate cortical bone
18
Q
What do the additional letters A,B,M mode stand for ?
A
- A-mode= amplitude mode
- B- mode= Brightness mode
- M-mode= time motion mode
- B mode mostly commonly used in clinical practice -> 2 d images
- A mode-> 1 dimensional image used in opthalmolgy
- M- used to asses cardiac valve motion
19
Q
What does CT stand for?
A
Computerised Tomography
20
Q
What is CT?
A
- Is an imaging modality that uses x-rays to create pictures of cross-sections of the body.
- X-rays are within a fan shaped x ray beam which rotates around the patient and sensitive detectors opposite which record the attenuated X-rays as they pass thru the body.
21
Q
What are ct scans made up of?
A
- Comprise of Scanning gantry - X-ray generator and curvlinear detector - opposite each other
- Pt couch
- Computer processor
- Display system
- Each rotation of the gantry produces an axial slice thru the patient
- . Modern scanners use helical rotation to allow continuous acquisition of the data
- The tissues that attenuate the beam to a higher degree, such as bone, appears denser on CT image than tissues such as muscle
- EAch tissue is assigned an attenuation coefficient ( in Hounsfield units, HU)
22
Q
What are the CT images made up of?
A
- Pixels- it not only presents a 2d image but a thickness (in VOXELS)
23
Q
What happens to the ct image before we see it?
A
- It has to be manipulated by changing the window levels & widths so that the variable areas of attenuation in the grey scale that we cannot determine by the naked eye can be seen
24
Q
What is ct used for ?
A
- Assessment of fractures
- With contrast to assess joints
- Bone densitometry
25
What tissues have a higher attenuation? How are this graded?
* Bone - appear denser on ct image- white cf areas of low attenuation- fat appear dark
* Each tissue is given a **ATTENUTATION COEFFICIENT** in **HOUNSFIELD UNITS** relative to the **Value of WATER** ( zero HU)
26
What are CT advantages ?
* **3D imaging and x sectional anatomical data**
* **Better than xrays** for soft tissue imaging but not as gd as MRI
* **Quantiative data on composition**
* Good for **assessing cortical bone**
* **Guide interventional proceedures**- biopsy
27
What are its disadvantages ?
* Significant use of **ionizing radiation**
* **Inferior soft tissue contrast** resolution to MRI
* **Claustrophic** for pt
28
What is the advantage of spiral CT?
* Allow rapid multi slicing as the gantry rotates continuously
29
What are the advantages of multidetector Ct?
* Multiple rows of detectors opposite side of X-rays source- so one rotation allow multiple slices thru the body to be imaged at once
30
What does MRI stand for?
* Magnetic resonance imaging
31
What is MRI?
* **An image modality using _STRONG MAGNETIC FIELDS_ and _EXCITATION RADIOFREQUENCY PULSES_ to the patient which results in _emission of RADIOFREQUENCY signals by the tissues_ of interest which are used to build up an image.**
32
What do MRI DETECT?
* **Hydrogen nuclei- PROTONS** in water
* Images reflect the _relative concentration of protons in tissues_ by measuring the **strength of signals from individual VOXELS In a slice of the patient.**
33
How do MRI work?
* Each proton spins like a top- aka **nuclear spin**
* Without a magnetic field the axes are **RANDOMLY ORIENTATED** so produced no net magnetic field.
* The MRI applies a **STRONG STATIC MAGNETIC FIELD -\> Protons AXIS ALIGN WITH LONG AXIS OF THE MAGNET/SCANNER -\> net magnetic field**
* The protons also wobble around their axis w a fixed frequency= **PRECESSION**
* A **RADIOFREQUENCY PULSE is applied for a few MILLISECONDS-\>** _protons realign at an angle to the longitudinal axis of the scanner_ & also causes precession of each proton to be pulled into step = phase with each other
* They induce currents in the recieving coils
* the changes in the currents/ signals are used to characterise the tissue
* When the pulse stops-
* the protons realign with the long axis of the scanner-\> long magnetization vector, it increases to max
* The precessional then fall out of step **(dephasing)**-\> the transverse magnetization vector to decrease from its max
34
Define T1 in an MRI image?
* The time taken for the **LONGITUDINAL MAGNETIZATION VECTOR to recover to 63% of its max value**
35
Define t2?
* The time taken for the **TRANSVERSE MAGNETIZATION VECTOR to fall to 37% of its max value.**
36
How are t1 and t2 images used?
* T1 and t2 vary for different tissues and therefore are used to form the image
* T1- fat bright signal - gd for anatomy
* T2- fluid has bright signal- gd for pathology
37
What is MRI used for?
* In orthopaedics
* Osteonecrosis
* Infection
* Trauma - occult fractures
* Tumours
* Regional disease- Rc pathology/ Intervertebral disc disease
38
What are its advantages?
* **Image in any plane**- multi planar
* **High contrast resolution**- gd soft tissue, bone marrow imaging
* **Non ionising radiation**
* Non invasive imaging of blood vessels
39
What are the disadvantages of MRI?
* Limited by contraindications- think clinic SHEET!!
* Absolute
* **pacemaker**
* **defribrillators**
* **cerebral aneurysmal clips**
* **vascular clips(\<2 wks old)**
* **metal in orbit**
* **internal hearing aids**
* **dorsal column stabilisers**
* Relative - 1st/2nd trimester of pregnancy , claustrophia, obesity, penile prostheis, children to sit within scanner
* Other
* **Not as good as Ct for imaging bone**
* **High cost of equipment**
* **Image artefact** - motion and ferromagnetic objects
* **Claustrophia**
40
What is a bone scan?
* **A image modality involving the intravenous injection of technetium 99m phosphonates, which localises at the sites of _OSTEOBLASTIC Activity_**
41
How does bone scanning work?
* Radionuclides are **UNSTABLE nuclei** with **neutrons excess or deficit**.
* The nuclei distingretate to form other atoms with the release of alpha, beta or gamma radiation.
* **Technetium 99m is a radionuclide with a short half life and pure _GAMMA emitter_**
* It is coupled with **methylene disphosphonate** and is Administered iv.
* It is **chemical absorbed onto hydroxyapitite crystals in bone.**
* It's uptake is reflection of **OSTEOBLASTIC activity.**
* within 24 hrs around 70% of administered dose is excreted thru kidneys hence bladder exposure to radiation must be minimised by pt hydration adn frequent micturition
42
How is the gamma radiation detected ?
* **Photo emission recorded by Scintillation gamma camera** ( contains *crystals of sodium iodide that absorb Tc 99 gamma rays*
* Signal is enhanced by **PHOTOMULTIPLIER TUBES and processed by computer**
43
Describe the 3 phases of a bone scan?
* 1)**Flow/ Dynamic phase**- immediate images **1-2mins**= arterial blood flow and hyper fusion
* 2) **Blood pool/ Equilibrium images**- images at _3-5mins_ showing **EC FLUID VOL**- so extent of bone and soft tissue hyperaemia
* 3) **STATIC/ Delayed images**- approx _4 hours_ after injection- when _soft tissue activity has cleared_, highlighting _skeletal activity_.anterior and post scans available.
44
What are bone scans used for?
* Assessment of occult bone pain
* Assessment of metastatic disease
* Infection - flow and blood pool- hot in cellulitis
* blood flow, pool and static phases all hot osteomyelitis
* Trauma- stress facture
* Tumour- osteoid osteoma osteoblastoma especially spine
* Arthritis- activity and extent
* Post-op- paeudoaethrothesis, painful prosthesis
* Assessment of Paget's disease
45
What are the advantages of bone scan?
* **Very sensitive** to increases in bone turnover
* **Good general survey** for skeletal pathology
* Helps to **localise local bone lesions** after which further imaging can occur
46
What are the disadvantages of bone scans ?
* **Poor spatial resolution**
* **Non specific appearance** for elucidating causes if increased activity
* may give **False negative in myeloma and lytic metastasis** due to _decreased blood flow or lack of osteoblastic response_
* **Relatively high radiation dose** - ESP to bone marrow and in children
47
What is SPECT?
* Single photon emission computer tomography
* **Applies tomography technology to radionuclide scanning**- enabling x sectional images to be obtained with enhances the conspicity of the lesion and hleos in their localisation
* the gamma head rotates 360 degrees around the body obtaining multiple planar images which are then reconstructed in sagittal, coronal and axial planes
48
What use is INDUM 111?
* It used to **label white cells and platelets** and can be used to **LOCALISE AREAS OF INFECTION AND INFLAMMATION**
49
What is the use of gallium 67?
* It binds to **plasma proteins **
* can be used to localise areas of inflammation e.g abscess and neoplasia
* frequently used in combo with technetium bone scan and is less dependent on vascular flow than a technetium scan
50
What is dexa?
* **Dual energy X-ray absorptiometry**
51
What is used for?
* To measure the **BONE MINERAL DENSITY in the central skeleton -FEMUR AND LUMBAR SPINE**
52
How does it work?
* It used **TWO X-RAYS OF DIFFERENT ENERGIES** **which are absorbed in different proportions by bone and soft tissues**
* The pt lies on the table for **2 mins** while **ap of femur **pt supine, ties together heels separated 25cm) and **lateral of spine**
* lower ribs cover L2 and iliac crests cover L4 on lateral so only **L3** maybe the only vertebra scanned.
53
What is a dexa scan used for?
* **Assessing bone mineral density in primary and secondary osteoporosis**- for diagnosis and monitoring - allows for fracture risk
* **Evaluating the effect of preventive interventions on metabolic diseases**- primary hyperparathyroidism
* **Measuring periprosthetic bone loss-** in pt with cementless thr- measure magnitude and rate of change in bm content
54
How are the results interpretated?
* **T score** when bone density values (g/cm3) are matched **FOR SEX AND RACE** and **COMPARED TO PEAK BONE MASS OF NORMAL ADULTS** =
* pt bmd - population bmd/ standard deviation of population bmd
55
What is the z score?
* When matched for **SEX, RACE AND *_AGE_*** =
* (pts bmd- pop age related bmd)/ Standard deviation of population age related bmd
56
Define osteoporosis ?
* is a progressive bone disease that is characterized by a decrease in bone mineral density which can lead to an increased risk of fracture.
* **diagnosed If the T score IS OVER -2.5 sd according to the WHO criteria** - kanis et al 1994
* Nb- **normal** t score is \>/** -1 **
* **Osteopenia** is a score **-2.5 to \<-1**
* **osteporosis \\<-2.5**
* severe oestoporosis = **\<-2/5 and presence of one or more fragility fx**
* **_In premenopausal women Z scores are used _**
57
What is z score used for?
* Compare pts **BMD** with the mean value for individuals of the **same age**
* A **LOW Z SCORE -= an aetiology other than age related bone loss**
* z score used in premenopausal women
58
What are the advantages of dexa scan?
* Assessment of fracture risk in osteoporosis and the need for treatment
59
What are the disadvantages of dexa scan?
* **Doesn't distinguish between CORTICAL and CANCELLOUS bone density**
* In the spine -AP PROJECTION fasley increase bmd by intervertebral oa- so prefer **LATERAL**
* _False high density values_ maybe seen with _fractured vertebra and calcification_
* _False low density_ values found if _posterior element surgical resection_
