General Radiology Flashcards
modality
A form of radiological imaging (ex. X-ray, CT, MR).
X-ray
a.k.a. “radiograph”
A 2D “single-slice” image created by bombarding the body region in question with a stream of high-energy electromagnetic radiation (X-rays) to develop an image.
Calcium absorbs X-rays the most, giving bones and calcifications a white appearance. Fat and other soft tissues absorb less and appear gray. Air absorbs X-rays the least, so lungs and other air pockets appear black.
X-rays are most commonly used to diagnose simple fractures, but can also be used to diagnose pneumonia and assess the placement of endotracheal tubes.
CT
Computed Tomography scan
a.k.a. “CAT scan”
A computer-processed combination of many X-ray images taken from different angles to produce a set of cross-sectional images (slices) of the scanned object.
As with X-rays, bone appears white, fat gray, and air black.
HU
Hounsfield Unit
a.k.a. “CT number”
Basically, a number that describes the absolute brightness of each data point in a CT scan. More precisely, it is a unit used universally in CT scans as a scale for describing radiodensity. It is named after Sir Godfrey Hounsfield, who helped develop CT scans. (And won a Nobel Prize for it!)
The scale runs from air (-1000 HU) through water (0 HU) to dense cortical bone (+1800 HU and up). Visually, the higher the HU the “brighter” the feature on the scan.
Generally, CT scan data is exported as 12-bit images which can store values between -1024 to +3071 HU. How these values are displayed is determined by the window and level values in a viewing program.
HU can be used to evaluate fat content in the liver, where an HU less than 40 can indicate a fatty liver.
kernel
a.k.a. “convolution algorithm”
Basically, the sharpness of a CT scan image. More precisely, it is an algorithm applied to the scan data to reduce noise and sharpen certain anatomical structures. This is done at the scanner.
Most commonly, the soft tissue kernel (closer to 30) is best at discerning soft tissue features, and lends a smooth appearance to bone. Bone kernel (closer to 60) causes soft tissue to appear grainy and gives bone a sharper appearance. Dual energy scans typically have several kernels applied to them, and the kernel that produces the desired effect can be chosen for post-processing.
MR
Magnetic Resonance
a.k.a. “MRI”
A type of scan in which nuclei of the hydrogen atoms in a patient are aligned in a strong, uniform magnetic field, absorb energy from tuned radio pulses, then emit radio signals. These signals are converted into images which appear as cross-sectional slices of the patient’s body.
The two basic types of MRI images are T1-weighted and T2-weighted images, often referred to as “T1” and “T2” images, in which different timings of radiofrequency pulses cause fat and water to enhance differently. In general, bone and air appear dark on MR scans. Another type is FLAIR, which causes fluids to appear dark.
MR scans may also be differentiated by magnetic field strength (ex. 1.5T and 3T, where T stands for “Tesla”). In general, the higher the magnetic field strength, the less noisy the images. (And the more expensive the scanner!)
Some patients may not be able to tolerate MR scans due to implants that contain magnetic metals. Non-magnetic metals are typically MR-safe, however (ex. gold, silver, titanium, copper, and stainless steel). However, unlike X-ray and CT scans, MR scans do not expose patients to ionizing radiation and are safer in that respect.
windowing
Basically, adjusting the brightness and contrast of a scan image. This adjusts features of interest to suit human vision, and subsequently, enables human analysis. (There may be a range of 4000 HU represented in a CT scan, but the human eye can only distinguish about 30 shades of gray!)
The brightness is determined by the “window width” (W) and the contrast by the “window level” (L). On a technical level, the width determines the range of HU’s visible, and the level is the HU around which this range is centered. (See diagram.)
Windowing is great for highlighting structures of interest at either end of the HU spectrum (ex. lungs vs. bone) and discriminating soft tissue from lesions (ex. increasing the contrast between tumor and liver tissue). Many presets are available to get you to the most appropriate window quickly (ex. “bone window”=W:1800 L:400, “lung window”=W:1500 L:-600), but fine-tuning is best done intuitively.
contrast
A substance that can be injected or injested in order to temporarily increase the visual contrast of key body structures. After contrast is administered, scans are performed at key time points when the contrast is passing through the regions of interest, or continuously to track its perfusion within regions of interest.
Most commonly, intravenous iodine-based contrast is used to enhance X-ray and CT scans. Intravenous gadolinium is used to enhance MR scans. Barium-sulfate-based contrast is ingested to enhance X-ray and CT images of the gastrointestinal tract.
contrast enhancement
Broadly, the degree to which a vessel or structure “enhances,” or brightens, when contrast passes through it.
Patterns of contrast enhancement may be used to differentiate tumors, which may have a stellate, homogenous, or heterogenous enhancement patterns when perfused with contrast-rich blood.
Please compare with “attenuation.”
attenuation
In general, the measurement of energy absorbed and deflected as it passes through a medium.
Attenuation can be used to describe the brighness of a feature on a CT scan. The greater the attenuation, the higher the HU. More specifically, attenuation in this case is the reduction of the intensity of an X-ray beam as it traverses matter. This may be caused by absorption or deflection (scatter) of X-ray photons, for example by bone or by a metal artifact.
ex. Bone attenuates more than soft tissue.
Attenuation can also be used to describe the blocking of signal in an MR scan. Certain scan settings can cause dark “attenuated” stripes to appear on the image.
Please compare to “contrast enhancement.”
image quality factors
A variety of factors may affect image quality:
- Implants that attenuate or scatter scan signal (pictured)
- Patient size (“body habitus”) can keep X-rays from penetrating, making medial features more noisy
- Movement (often due to heartbeat, breathing)
- Magnetic strength (1.5T vs. 3T)
- Amount of contrast (can be inadequate)
- Timing of contrast (may scan at suboptimal points in its circulation)
artifact
Generally, something seen in a scan that is due to a quirk of the imaging modality, rather than something that exists in reality.
Metal streak artifact can severely obscure CT images with light and dark rays that extend from metal implants, which can be anything from a joint prosthesis to dental fillings. It can create “blown-out” regions where no amount of windowing can reveal the obscured anatomical structures. If regions surrounding metal hardware need to be assessed, this is best mitigated by using a lower CT energy level or a dual energy scan (with post-processing).
Metal artifact can obscure MR scans as well, where it typically appears as a blacked-out area.
Stair-step artifact (pictured) can occur when a heart scan is not well-gated. This means that each pass captures the heart at a slightly different phase of the heartbeat, resulting in a “stair-step” appearance.
Movement can cause bothersome artifacts across modalities as well. Breathing can result in abdominal organs that move up and down across perfusion series, and the movement of bowel gas can cause a checkerboard-like artifact that can obsure prostate MRs.
hardware
Loosely refers to implants, prosthetics, screws… any “hard” man-made material implanted into a patient’s body. Metal hardware is notorious for causing streak artifact on CT scans.
angiogram
a.k.a. “CTA” or “MRA”
A CT or MR scan which visualizes blood vessels at a high resolution. This image data is used to create 3D models of the vessels to evaluate them for aneurysms, stenosis, dissections, and other vascular abnormalities.
CTAs require contrast, whereas MRAs may or may not include contrast.
CEMRA
Contrast-Enhanced MRA
An MRA done with contrast, timed to capture the contrast when it is passing through the arteries but has not reached the veins.
If available, CEMRA is preferable to TOF for 3D post-processing, as it displays the size and shape of aneurysms and other abnormalities more accurately. However, it can include an annoying amount of venous contamination (contrast in the veins) that can hide 3D findings and needs to be sculpted away.
TOF
Time of Flight
A type of MRA that uses speed of blood flow, rather than contrast, to visualize arteries. This eliminates issues with venous contamination, but can omit key findings where blood flows more turbulently (ex. turbulent blood flow within an aneurysm sac causes the aneurysm to appear smaller on the TOF).
perfusion
A type of scan that visualizes the passage of intravenous contrast through a region over time.
Perfusion scans are typically post-processed to generate color maps and other visuals that reveal how contrast perfuses lesions (ex. brain tumors) differently than healthy tissue. Perfusion data can reveal brain tumors, determine the degree of brain damage from a stroke, and differentiate benign from malignant renal masses.
multiphase
A scan performed across multiple timepoints of a biological process, for example a series of CTs showing the journey of contrast from the bloodstream to the bladder, or an angiogram performed across the cardiac cycle.
T1 MRI
A type of MRI where the timing of radiofrequency pulse sequences causes fat to appear bright.
T2 MRI
A type of MRI where the timing of radiofrequency pulse sequences causes BOTH water AND fat to appear bright.
FLAIR MRI
Fluid-Attenuated Inversion Recovery MRI
A type of MRI with an inversion recovery set to null fluids (it makes fluid appear dark).
series
a.k.a. “sequence”
A set of images within a study, usually representing some specific protocol.
ex. The study was missing a CEMRA series since the patient could not tolerate contrast, so it contained a TOF series instead.
slice
a.k.a. “image”
An image within a CT or MR series. They are usually numbered. They are most commonly oriented in the axial plane, but can also be coronal, or less-commonly, sagittal. The accuracy of the plane orientation may vary depending on the position of the patient in the scanner.
ex. The calcification was visible on slice #173.
