ALARA – (As Low As Reasonably Achievable) A concept to keep the radiation dose low but one should make sure the test remains diagnostic.
Attenuation – Brightness of an object, based upon density. Typically measured by use of the
Hounsfield unit – (HU), which is a scale developed by Sir Godfrey Hounsfield that attributes brightness of water as a HU of 0, and less dense objects being negative (i.e., air is typically −1000 HU), and more dense objects being positive (i.e., metal is typically +1000 HU).
Beam Hardening – An artifact with effects on image accuracy primarily due to high–density metal or calcium situated near a low-density structure (i.e., soft-tissue or lumen), which attenuates the X-ray signal from disproportionately low-energy X-rays.
Blooming – An artifact on CT which is caused by severe calcification or metal, which causes the bright object to appear larger than it is (due to partial voluming).
Collimation – The amount of tissue imaged on one rotation of the detectors, calculated as thenumber of detectors multiplied by the slice thickness (i.e., 64 detectors at 0.625 = 40 mm).
Collimator – Shapes the X-ray beam. This controls the width, detector configuration, quality,and position of the X-ray beam.
Contrast to Noise Ratio – A marker of image quality on CT, with a goal of high contrastenhancement to low noise. The worse the ratio, the harder the study is toi/nterpret. Similar tosignal to noise ratios.
Convolution kernel – Defined as the image processing filter applied to the raw data to yield afinal scan image. The sharpness of the final image is most directly influenced by the type offilter employed.
CT Dose Index (CTDI) – Dose delivered to a standard reference phantom.
Dissipation – How fast the tube can cool down, which is a limiting factor for image quality.
Effective Dose – Reported in millisieverts (mSv), is a measure of the overall detrimental biologicaleffect of a given radiation exposure. It is calculated by weighting the concentrationsof energy deposited in each organ from a given radiation exposure, taking into account typeof radiation and the potential for organ-specific damage in a reference individual.
Field of View – The scanned field of view represents the entire object scanned within the gantry;the displayed field of view is defined as the angular size of the displayed scan on thethree-dimensional matrix.
Negative Predictive Value – The percentage of patients with negative results that are actuallynegative (i.e., free of the disease or condition). The higher the negative predictive value, themore accurate the test is at excluding patients that do not have the disease or condition.
Partial Volume Averaging – An artifact on CT caused by objects being incompletely imagedon an individual slice of data. A small object (i.e., 0.3 mm foci of fat in the vessel) may be seenon an individual slice (0.6 mm), but may be somewhat obscured due to averaging with higherdensity objects around it. Thus, since most imaging on CT is based upon maximal intensityprojection, the brightest objects (calcium, metal) tend to dominate lower density objects(lumen, non-calcified plaque). Newer scanners with the ability to obtain thinner slices thereforehave less problems with partial voluming.
Peak Tube Voltage (kVp) – Is the energy of the X-rays produced, measured in kilovolts.
Pitch – Is defined as the ratio of table travel to the beam collimation.
Pixel – A two dimensional measure describing the x- and y-axis resolution of an individualdatapoint. Using a 512 × 512 matrix, current CT scanners can achieve resolution of an individualdatapoint of 0.35 × 0.35 mm.
Positive Predictive Value – The percentage of patients with positive test results who are actuallypositive (i.e., have the disease or condition). The higher the positive predictive value, themore accurate the test is at detecting/diagnosing the disease of interest.
Prospective ECG Gating – Relies on the scanner initiating imaging only during a pre-specifiedinterval of the cardiac cycle (otherwise known as step and shoot).
Retrospective ECG Gating – Involves a continuous spiral (helical) feed and scan wherein theentire heart volume is covered continuously. Data acquisition occurs from all phases of thecardiac cycle.
Sensitivity – The percentage of actual positives which are correctly identified by the test(or the ability of the test to detect disease at a particular level).
Signal to Noise Ratio – A marker of image quality on CT, describing the ratio of the imagesignal (attenuation or brightness) to the background noise level. Increasing radiation doses(especially kVp) will increase the signal and decrease the noise.
Spatial Resolution – Refers to the ability of an imaging modality to detect two distinct objectsin space as separate objects, and is expressed in units of distance.
Specificity – The percentage of negatives which are correctly identified (or the ability of thetest to detect the disease or condition of interest).
Temporal Resolution – Refers to the ability of an imaging modality to detect two distinctevents in time as separate events, and is expressed in units of time.
Tube Capacity – How much heat a tube can store safely.
Tube Current – It is the number of X-rays produced, measured as milliAmperes (mA).
Voxel – A three dimensional datapoint on CT (pixel with z axis added). An isotropic voxeldescribes the ability to have all three dimensions with equal spatial resolution (i.e., 0.35 ×0.35 × 0.35 mm), allowing free rotation of the three-dimensional image without distortion.