Radiology basics Flashcards
(31 cards)
Noise
undesirable background variations in signal not arising from the energy source — can be random quantum noise or noise from system electronics — (the lower the signal-to-noise ratio (SNR) the greater the image graininess and mottling and the poorer the image quality)
Temporal Resolution
— the time it takes to acquire an image — (the greater the temporal resolution [ie, the faster the image acquisition], the lower the likelihood that patient motion will cause artifactual image blurring) – all modalities except for MRI and Nuclear Medicine now have fast sub-second image acquisition times that make motion artifact uncommon
Spatial Resolution
the ability to distinguish two tiny adjacent structures in an image as separate — (the greater the spatial resolution, the smaller the field of view, the larger the image matrix, and the smaller the image pixel size) – all modalities except for Nuclear Medicine now have superb sub-millimeter spatial resolution
Tissue Contrast Resolution
the ability to distinguish two adjacent structures in an image that vary only slightly in composition — (the greater the tissue contrast resolution, the greater the conspicuity of different adjacent healthy and/or pathologic tissues)
Contrast agents
Barium and iodine are large elements that absorb and scatter more incoming X-ray photons than most body tissues, thereby causing image brightening in proportion to their concentration on images from modalities employing X-rays
Gadolinium is a paramagnetic element (due to several unpaired electrons) that distorts the local magnetic field, thereby causing image brightening proportion to their concentration on T1-weighted MRI images (but not T2-weighted MRI images)
Barium Contrast Agents
Administered orally or rectally for GI tract fluoroscopy studies or body CT studies
NOT injected elsewhere because NOT cleared by kidneys or liver
Imaging Contrast Agent Safety
Safe in GI tract – no allergic reactions
Just don’t inject it IV or anywhere else!
Iodinated Contrast Agents
Injected intravascularly for CT, CT angiography (CTA) or conventional angiography studies
Administered orally or rectally for GI tract fluoroscopy studies or body CT studies
Injected elsewhere in body (spinal canal, biliary tree, urinary tract, joints, etc.) for CT or fluoroscopy studies
Cleared by kidneys
Imaging Contrast Agent Safety
Not given if intravascularly if severe renal insufficiency (estimated GFR < 30 ml/min)
Nephrogenic systemic fibrosis (NSF) – a rare potentially fatal fibrosing condition of the skin and organs that is only encountered following gadolinium administration in patients with severe renal insufficiency (essentially no new cases of this are now occurring in developed world due to careful pre-scan renal function screening)
Unlike iodinated contrast, is NOT nephrotoxic
Gadolinium Contrast Agents
Injected intravascularly for MRI or MR angiography (MRA) studies
Injected elsewhere in body (spinal canal, biliary tree, urinary tract, joints, etc.) for MRI studies
Cleared by kidneys (some niche agents also cleared by liver)
Imaging Contrast Agent Safety
Not given if intravascularly if severe renal insufficiency (estimated GFR < 30 ml/min) due to nephrotoxicity
Contrast-induced nephropathy (CIN) can develop due to the nephrotoxicity of intravascular iodinated in cases of severe renal insufficiency
Good IV hydration prior to contrast delivery is the only effective method of preventing or minimizing CIN
Adverse Contrast Reactions
Never with barium, quite rare with gadolinium, less rare with iodinated contrast
Rare after intravascular gadolinium or iodine administration, but extremely rare after administration via other routes
Not true hypersensitivity anaphylactic reactions (no IgE response), so referred to as idiosyncratic or anaphylactoid adverse reactions
Risk Factors for Both Gadolinium & Iodinated Contrast Reactions
Prior adverse reaction: 3.5 – 7 fold increased risk c/w general population
Asthma, hay fever or multiple food or medicine allergies: 1.5 – 3 fold increased risk c/w general population
Shellfish allergy is NOT a risk factor (even though shellfish accumulate iodine)
Prevention of Repeat Reaction by Steroid Premedication
Patients who have had a prior moderate or severe adverse contrast reaction can be pre-medicated with steroids (prednisone or methylprednisolone) to lessen (but not totally eliminate) the chance of a repeat reaction
ALARA
Imagers follow the ALARA principle for ionizing radiation modalities in medical imaging, meaning we strive to keep the radiation dose to patients “As Low As Reasonably Achievable”
CT, fluoroscopy, and X-ray machines are calibrated to emit the lowest patient radiation dose that provides images with “clinically acceptable” noise (rather than emitting more radiation to obtain the prettiest images with the highest SNR)
Mammography
A high definition subtype of 2D planar radiography of the breasts
The breast is squeezed and flattened between two compression paddles to spread it out and minimize superimposition of tissues before images are acquired
Radiation dose is low enough to justify annual exams in women 40 years of age and older
Fluoroscopy
Fluoroscopy (colloquially referred to as fluoro) allows for real time X-ray imaging by producing a continuous low flux stream of X-ray photons that the operator controls using a foot pedal (note that photon flux = number of photons emitted per unit time)
The operator can choose to record real-time video clips and/or still-life “spot images” during a study
The name fluoroscopy comes from the fluorescent screen early fluoroscopes employed – an “image intensifier” amplifies the faint signal enough to afford good quality real-time moving pictures
Uses of fluoroscopy include
Gastrointestinal tract studies with Barium or iodinated contrast
Genitourinary tract studies with iodinated contrast
Angiography with IV iodinated contrast
Real-time visualization during other procedures by interventional radiologists, cardiologists and surgeons (often employing iodinated contrast)
Ultrasound (US)
Ultrasound imaging is also known as sonography — and sonogram is another term for an ultrasound study
Sonographer is the term for an US technologist, and sonologist is the term for the physician interpreter
Medical sonography employs very high frequency sound waves (approximate range of 2-18 mHz), above our auditory threshold
Ultrasound scanner transducers (also known as probes) contain piezoelectric crystals and make use of the piezoelectric effect to both generate high frequency sound waves and detect returning echoes (transducers serve as both transmitters and receivers)
Sound waves generated by transducers are impeded and attenuated to various degrees as they penetrate various tissues
Types of acoustical impedence include scattering, refraction, absorption and/or reflection
The reflected acoustical echoes are detected by the transducers and this data is processed by ultrasound unit computers into images
Different substances display different levels of echogenicity – the greater the transmission (ie, the less the acoustical impedance) of sound waves through a tissue, the less echogenic (ie, the darker) the tissue appears on the scanner monitor — for example:
Water and simple fluid — nearly all sound waves are transmitted through it, so it has a completely black (termed anechoic) appearance; and since the sound waves are nearly all transmitted, the tissues deep to the fluid are artifactually brighter than they otherwise would be (referred to as increased through transmission)
Various soft tissues — gray appearance ranging from dark gray if little acoustical impedance (termed hypoechoic) to brighter gray if greater acoustical impedance (termed hyperechoic) in appearance
Gas and calcified structures — virtually all sound waves are scattered and reflected, yielding a bright white front interface (very hyperechoic) appearance and black (anechoic) shadowing out of everything posterior (deep) to the front interface (termed acoustic shadowing)
The higher the transducer acoustical frequency, the greater the image spatial resolution, but the lower the depth of tissue penetration, and the smaller the field of view (and vice versa)
Magnetic Resonance Imaging (MRI)
MRI exploits the phenomenom of nuclear magnetic resonance, in which atomic nuclei in an external magnetic field absorb and re-emit electromagnetic energy that is transmitted at their specific resonance frequency (remember this from your college chemistry labs?)
Clinical MRI scanners with powerful magnetic fields transmit radio wave “pulses” at the resonance radiofrequency of the single protons in hydrogen nuclei, thereby exciting all of the hydrogen protons in the tissues within the area of interest
Hydrogen is the target atom because hydrogen is in water and all organic molecules, and since human bodies are 60% water by weight, tissue water content is the most important factor in determining the MRI appearance
As the hydrogen protons “relax” from their excited state back toward their baseline state following each radio wave pulse, the radiofrequency energy signal they emit is detected as “echoes” by nearby receiver antennae
Both the T1 (longitudinal) relaxation time and the T2 (transverse) relaxation time of tissues depend upon the amount of water and the composition of the organic molecules within them
Thus different tissues will differ in appearance on T1-weighted and T2-weighted MRI images based upon their proportions of water and various organic molecules ( lipids, carbohydrates, amino acids, etc.)
Scanning is generally performed in all 3 planes, with various combinations of T1-weighted and T2-weighted “pulse sequences”
Differentiating T1 MRI vs T2 MRI vs CT Images
CT images
Yield excellent anatomic detail but only modest tissue contrast resolution (like T1-weighted MRI)
Simple fluid (CSF, bile, urine, ascites, etc.) ranges from dark gray to nearly black in appearance
Cortical bone is always brighter than trabecular bone for CT but never for MRI – calcium has a higher atomic number and absorbs or scatters more CT X-ray photons than tissues composed of organic molecules
Fat is dark black in appearance (assuming standard soft tissue windowing)
Pathology is generally inconspicuous and only slightly darker than normal tissue
Intravenous iodinated contrast administration (C+) enhances tissue contrast resolution, and often renders pathology brighter and more conspicuous (like T1-weighted C+ MRI)
T1-weighted MRI images
Yield excellent anatomic detail but only modest tissue contrast resolution (like CT)
Simple fluid (CSF, bile, urine, ascites, etc.) is always very dark (nearly black) in appearance
Cortical bone is always dark (because very few hydrogen protons in bone)
Body fat and fatty yellow bone marrow are bright (unless a fat suppression technique is used)
Pathology is generally inconspicuous and only slightly darker than normal tissue
Intravenous gadolinium contrast administration (C+) is only employed for T1-weighted sequences (because it primarily affects tissue T1 relaxation times, not T2 relaxation times) – it enhances tissue contrast resolution, and often renders pathology brighter and more conspicuous (like C+ CT)
So, if doing a contrast enhanced MRI scan, the tech runs standard pre-contrast T1-weighted and T2-weighted pulse sequence, then injects IV gadolinium, then runs additional post-contrast T1-weighted sequences
T2-weighted MRI images
Yield much better tissue contrast resolution but mildly less anatomic detail than T1-weighted images (or CT)
Simple fluid (CSF, bile, urine, ascites, etc.) is always bright white
Cortical bone is always dark
Body fat and fatty yellow bone marrow have variable signal intensity
Most tumors and inflammatory processes have a higher water content than normal and are therefore brighter than healthy tissues
Pathology is often more conspicuous on T2-weighted MRI images (sometimes “light bulb bright”) than on non-contrast T1-weighted MRI or CT images
Fat-Suppressed (FS) MRI Imaging
Fat suppression (FS or “fat sat”) is commonly employed for MRI pulse sequences to improve tissue contrast resolution by suppressing the relatively bright signal from body fat and fatty yellow bone marrow, causing these tissues to become extremely dark
Suppressing signal from fat can be helpful because pathological tissue stands out more conspicuously against a dark background (this phenomenon is similar to the image clarity improvement when the lights go down in a movie theater or in a radiologist’s reading room )
Fat suppression is used most routinely in MR angiography (MRA) and orthopedic MRI, but it is also often used in body and spine MRI
When the fat is dark on an image, the study must be either a CT or a fat-suppressed (FS) MRI scan
Rely on the appearance of the skeleton to distinguish between CT & MRI (bright cortical bone for CT – dark cortical bone and bone marrow for FS MRI)
Nuclear Medicine (NM)
With the other cross-sectional imaging modalities we obtain great anatomical spacial resolution and great temporal resolution (aside from most MR studies) but little or no physiological information
And with radiography, fluoroscopy, and CT we detect high energy photons (X-rays) beamed clear through the patient
In contrast, in NM imaging the patient is made radioactive and high energy ionizing photons (gamma rays) emanating from the patient are detected by gamma cameras and processed into images
With NM imaging we obtain great physiological information but much poorer anatomical spacial resolution (about 8-10 mm) and poor temporal resolution (several minutes per image, about 30-60 minutes per scan)
Nuclear Medicine Gamma Cameras
The gamma rays emanating from the patient are detected by a nuclear medicine scanner known as a gamma camera (generally with two rotatable detector heads positioned opposite one another)
Scintillators (such as sodium iodide crystals) are used in the detector heads – they give off light when struck by a gamma ray (via radio- luminescence), the light results in an electrical charge (via the photovoltaic effect) within the detector, and this electricity flows to the computer for image generation
Nuclear imaging is also known as nuclear scintigraphy because scintillators are used, and nuclear studies are also known as scintigrams (ie, a “nuclear bone scan” can also be called a “bone scintigram”)
Each image is completed after the gamma camera detects a specified number of photons (on the order of 500,000 - 1,000,000 counts)
Standard imaging right vs left labeling does not always hold with NM images, so don’t worry about image orientation for now
3D NM – SPECT Scintigraphy
In Single Photon Emission Computed Tomography (SPECT) the dual gamma camera heads opposite one another are incrementally rotated through 180 degrees around the patient, pausing to collect thousands of gamma photon “counts” for a few minutes every few degrees of rotation
Images are generally reconstructed in the standard axial, sagittal, and coronal planes (remember that imaging is 3D if multi-planar reconstructions can be generated from the data)
Spatial resolution is better for SPECT than for planar scintigraphy
Positron Emission Tomography (PET)
PET imaging is a subtype of nuclear medicine (NM) that involves administration of positron-emitting radionuclides which result in 2 gamma rays being given off simultaneously in opposite directions These cyclotron-created radioisotopes are highly unstable and have very short half-lives – Fluorine 18 in F-18 fluorodeoxyglucose (FDG) is most often used A positron (positively charged electron) emitted by nuclear decay is invariably “annihilated” very quickly by colliding with an electron of a nearby atom -- during this annihilation event the mass of the positron and electron is converted to pure energy in the form of two gamma rays (in keeping with the law of conservation of matter and energy) that shoot off in exactly opposite directions
PET scanners have rings of “coincidence” scintillation detectors along the gantry that only record “counts” when two photons simultaneously strike the detectors 180 degrees apart, indicating that they came from the same positron / electron collision (in contrast, in SPECT and planar scintigraphy only single photon strikes are recorded)
Spatial resolution is better for PET than for SPECT due to this coincidence detection
Spatial resolution is best for all non-nuclear studies > PET > SPECT > planar nuclear studies
Image Intensity Terminology
X-ray or Fluoro: darker = radiolucent (lucent) – destructive-appearing lucent bone lesion referred to as “lytic”
X-ray or Fluoro: brighter = radio-opaque (opaque) – abnormally bright bone referred to as “sclerotic”
CT: darker = hypodense or lower attenuation (having lower Hounsfield Unit values) – destructive-appearing lucent bone lesion referred to as “lytic”
CT: brighter = hyperdense or higher attenuation (having higher Hounsfield Unit values) – abnormally bright bone referred to as “sclerotic”
MR: darker = lower signal
MR: brighter = higher signal
Apophysis
A normal bony outgrowth that arises from a separate secondary ossification center that eventually fuses with the main bone
It is a site of tendon or ligament attachment(s)
Unfused apophyses can easily be mistaken for fractures
Examples are the coracoid and acromion processes of scapula and the greater tuberosity, medial epicondyle, and lateral epicondyle of humerus
Epiphysis
A secondary ossification center at the end of a long bone that forms part of a synovial joint
Tuberosity / Tubercle
A bony prominence at a major tendon attachment site that does not arise from a separate secondary ossification center
Examples include the lesser tuberosity (subscapularis) and deltoid tuberosity (deltoid) of the humerus and the radial tuberosity (biceps brachii) of the proximal radius
