NMTCB(RS) key terms

A radioactive drug that is given to a patient to diagnose or treat disease. It combines a radionuclide with a carrier molecule that determines where the activity localizes in the body.

An unstable form of an element that emits radiation as it decays. In nuclear medicine, the emitted gamma rays or positrons are detected to form images or to deliver therapy.

The time required for half of the radioactive atoms in a sample to decay. Physical half-life is a fixed property of the radionuclide, while biologic half-life describes removal from the body.

The combined result of physical decay and biologic clearance, describing how fast activity leaves a patient's body. It is calculated from the physical and biologic half-lives and is always shorter than either one.

Removal of a radiopharmaceutical from the body by physiologic processes such as renal excretion, metabolism, or exhalation. It determines background activity and the timing of imaging after administration.

The comparison of activity concentration in the organ of interest versus surrounding tissues. A higher ratio improves lesion visibility, and it can be raised by waiting for clearance or by patient preparation.

An imaging device that detects gamma rays emitted from a patient. It uses a collimator, a scintillation crystal, and photomultiplier tubes to convert light flashes into position signals that form a planar image.

A lead shield with holes placed between the patient and the detector. It restricts the direction of incoming gamma rays so that detected events correspond to a known source location, at the cost of sensitivity.

Gamma rays that pass through the lead walls between collimator holes instead of traveling along a hole. This degrades spatial resolution, especially with high-energy radionuclides and low-energy collimators.

A vacuum tube in a gamma camera that converts light from the scintillation crystal into an electrical signal and amplifies it. The pattern of tube signals is used to calculate event position and energy.

A selected range of pulse heights accepted by the camera as valid events for a particular radionuclide. It is centered on the photopeak to exclude scattered radiation and improve image contrast.

The peak in a gamma-ray energy spectrum representing full absorption of a photon in the detector. Camera energy windows are centered on this peak for the radionuclide being imaged.

Photons that change direction and lose energy after interacting with tissue or shielding. If accepted, scattered photons add counts in the wrong location and reduce image contrast.

The ability of an imaging system to distinguish two closely spaced objects as separate. It is affected by collimator design, distance from the collimator, and the intrinsic resolution of the camera.

The number of counts recorded per unit of administered activity for a given imaging system. Higher sensitivity allows shorter scans or lower doses but often trades off against resolution.

A region on an image that shows less activity than surrounding tissue, appearing as a cold spot. It may represent a cyst, infarct, avascular mass, or an area displaced by another structure.

Dynamic imaging performed immediately after a bolus injection to show blood flow through vessels and organs. It is used to evaluate perfusion, shunts, and vascular patency.

The distribution of a radiopharmaceutical within the body based on its physiologic or biochemical behavior. Understanding the mechanism helps interpret normal uptake patterns and recognize abnormal findings.

An ionization chamber device used to measure the activity of a radiopharmaceutical before administration. It must be checked for accuracy and constancy and corrected for radionuclide and geometry.

The principle of keeping radiation exposure as low as reasonably achievable while still obtaining the needed diagnostic information. It guides shielding, time, distance, and dose-reduction practices.

Compensation for the loss of signal as photons travel through tissue. In PET/CT and SPECT/CT, the CT image is used to estimate tissue density so that deep structures are not falsely underestimated.

A scaled number expressing CT attenuation relative to water, which is defined as zero. Air is about -1000 and dense bone is several hundred to over 1000, allowing tissue characterization.

The range of CT numbers mapped to the visible gray scale on a display. A narrow width increases contrast between tissues with similar attenuation, while a wide width shows more structures at once.

The center CT number of the displayed gray scale. Changing the level brightens or darkens the image overall, while width controls contrast.

In helical CT, the table distance traveled per gantry rotation divided by the beam collimation width. Higher pitch speeds coverage but can affect dose and image noise.

CT acquisition in which the x-ray tube rotates continuously while the table moves, producing a continuous volume of data. It enables fast coverage and multiplanar or three-dimensional reconstruction.

Reformatting axial CT data into coronal, sagittal, or oblique planes. Thin-slice volumetric data are required for smooth, diagnostically useful reformats.

The reconstructed thickness of each CT image in the z-axis. Thinner slices improve spatial resolution and reformat quality but increase noise and data volume if other factors are unchanged.

Volume computed tomography dose index, an estimate of the radiation dose delivered to a standard phantom per scan. It is displayed on the scanner and used to compare protocols and track dose.

Dose-length product, calculated as CTDIvol multiplied by the scanned length. It reflects total radiation output for a study and can be converted to effective dose using a body-region factor.

Artifact caused by lower-energy x-rays being absorbed preferentially as the beam passes through tissue. It produces streaks or dark bands, often between dense structures such as bone.

Linear bands across a CT image usually caused by high-attenuation objects, patient motion, or photon starvation. It can obscure anatomy and is reduced by technique adjustments or reconstruction algorithms.

Blurring or misregistration on CT caused by patient movement, breathing, or cardiac motion. It degrades image quality and can create false registration errors when fused with functional images.

A combined scanner that acquires positron emission tomography and computed tomography in one session. The CT provides anatomic detail and attenuation correction for the PET emission data.

Increased CT attenuation of tissues after intravenous iodinated contrast administration. Timing and phase determine which vessels and organs are highlighted for interpretation.

A CT technique in which a monitoring scan detects arrival of contrast in a target vessel and triggers the diagnostic acquisition. It improves timing for arterial-phase studies.

A CT image reconstruction method that repeatedly compares measured data with a modeled image to reduce noise. It allows lower dose while maintaining acceptable image quality compared with filtered back projection.