- Hounsfield unit (HU)
- The standardized CT number for tissue attenuation: water = 0 HU, air = −1000 HU, dense cortical bone ≈ +1000 HU or more. Fat is about −100 to −50; soft tissue +30 to +60.
- CTDIvol
- Volume CT Dose Index (mGy) — the average radiation dose within the scanned volume, normalized for pitch. The primary dose metric displayed on the console for a single scan series.
- DLP (dose-length product)
- CTDIvol × scan length, in mGy·cm. It estimates the total radiation delivered over the whole scan and is used to estimate effective dose (DLP × a region-specific k factor).
- Pitch
- Table travel per gantry rotation ÷ total nominal beam width. Pitch > 1 spreads the beam (faster scan, less overlap, lower dose); pitch < 1 overlaps the beam (more dose, less noise).
- ALARA
- As Low As Reasonably Achievable — keep every CT radiation dose to the patient and staff as low as possible while still producing a diagnostic image.
- Window width (WW)
- The range of Hounsfield units displayed as shades of gray. A wide window (e.g., lung/bone) shows many HU values with low contrast; a narrow window shows few HU values with high contrast.
- Window level (WL)
- The center Hounsfield value of the displayed window — it sets the midpoint of the gray scale (brightness). Choose WL near the HU of the tissue of interest (e.g., ~40 for brain).
- eGFR
- Estimated glomerular filtration rate — a measure of kidney function used to screen before iodinated IV contrast. A low eGFR raises the risk of contrast-associated kidney injury, so protocols are adjusted.
- Nonionic contrast media
- Low-osmolality iodinated contrast that does not dissociate into ions. It is far better tolerated than ionic (high-osmolality) agents and is the standard for intravascular CT injection.
- Metformin and iodinated contrast
- Metformin is renally cleared; if contrast impairs kidney function, metformin can accumulate and (rarely) cause lactic acidosis. Follow ACR/FDA guidance — assess renal function and hold metformin per protocol in at-risk patients.
- Extravasation
- Leakage of injected contrast into the soft tissue around the IV instead of the vein. Stop the injection, elevate and apply cold/warm compresses per policy, assess for compartment syndrome, and document.
- Premedication for contrast allergy
- A patient with a prior iodinated-contrast reaction who still needs contrast may receive a corticosteroid + antihistamine premedication protocol (per ACR) to reduce the risk of a repeat reaction.
- Beam hardening artifact
- As the polychromatic x-ray beam passes through dense tissue, low-energy photons are absorbed first, raising the mean beam energy. This produces dark streaks/cupping, classically between dense bone (posterior fossa) or metal.
- Bowtie filter
- A shaped filter at the tube that equalizes x-ray intensity across the fan beam — more attenuation at the periphery (thin body) and less centrally — reducing peripheral patient dose and beam-hardening artifact.
- Tube current modulation (AEC)
- Automatic adjustment of mA based on patient size/attenuation along the scan (angular and z-axis). It delivers uniform image quality at the lowest dose; brand names include SmartmA, CARE Dose4D, SURE Exposure.
- MDCT (multi-detector CT)
- A scanner with multiple rows of detectors along the z-axis, acquiring several slices per rotation. More detector rows mean faster coverage, thinner slices, and isotropic volumetric data.
- Isotropic voxel
- A voxel with equal dimensions in all three axes (x, y, z). Isotropic data allow high-quality multiplanar (MPR) and 3D reformations in any plane without loss of resolution.
- MPR (multiplanar reformation)
- Reconstructing axial volumetric data into coronal, sagittal, or oblique planes. Requires thin, overlapping slices (ideally isotropic) for the best results.
- MIP (maximum intensity projection)
- A post-processing display that projects the highest-attenuation voxels along a ray — used to show contrast-filled vessels in CT angiography.
- Helical (spiral) CT
- Continuous tube rotation while the table moves through the gantry, acquiring a volume of data in one breath-hold. Enables thin slices, fast coverage, and overlapping reconstructions.
- Iterative reconstruction
- A reconstruction method that repeatedly refines the image by comparing it to the raw projection data, reducing image noise and allowing lower-dose scanning compared with filtered back projection.
- Filtered back projection (FBP)
- The classic CT reconstruction algorithm that back-projects filtered projection data into the image. Fast and simple, but noisier at low dose than iterative methods.
- Reconstruction kernel (algorithm)
- The mathematical filter applied during reconstruction. A 'sharp/bone' kernel boosts spatial resolution but increases noise; a 'smooth/soft-tissue' kernel lowers noise but blurs fine detail.
- Bolus tracking
- A scanning technique that places an ROI in a target vessel and automatically triggers the scan when contrast enhancement reaches a set HU threshold — timing the scan to peak vascular opacification.
- Timing bolus (test bolus)
- A small test injection of contrast scanned repeatedly at one level to measure time-to-peak enhancement, then used to set the optimal scan delay for the diagnostic acquisition.
- Cardiac CT — ECG gating
- Synchronizing acquisition (prospective) or reconstruction (retrospective) to the cardiac cycle (usually diastole) to freeze heart motion. Prospective gating gives lower dose; retrospective allows functional/phase data.
- Beta-blocker before coronary CTA
- Given to lower and stabilize the heart rate (target ~ <60 bpm), reducing cardiac motion and improving coronary artery image quality on gated cardiac CT.
- Non-contrast CT for acute stroke
- The first study in suspected stroke — it is performed to exclude intracranial hemorrhage before any thrombolytic therapy or contrast study.
- CT perfusion — CBF
- Cerebral blood flow, a key parameter measured in brain CT perfusion to distinguish salvageable ischemic penumbra from infarcted core in acute stroke.
- CT pulmonary angiogram (CTPA)
- Contrast-enhanced chest CT timed to the pulmonary arterial phase (~ peak pulmonary artery enhancement) to detect pulmonary embolism.
- Non-contrast CT for renal stones
- Suspected renal/ureteral calculi are imaged without contrast, because contrast in the collecting system would obscure (or mimic) the high-attenuation stones.
- Multiphase liver CT
- Imaging the liver in arterial, portal-venous, and sometimes delayed phases to characterize lesions by their enhancement pattern (e.g., arterial hypervascularity of hepatocellular carcinoma).
- CT urography (CTU)
- A multiphase study (non-contrast, nephrographic, and excretory phases) that opacifies the collecting systems, ureters, and bladder to evaluate the urinary tract.
- CT enterography
- Abdominal CT using a large volume of neutral (low-density) oral contrast to distend the small bowel, improving detection of bowel wall and mucosal disease (e.g., Crohn's).
- Scout / topogram (localizer)
- The preliminary low-dose projection image acquired first to plan the scan range, centering, and AEC reference — not for diagnosis.
- Dual-energy CT (DECT)
- Acquiring data at two x-ray energies to differentiate materials by their energy-dependent attenuation — e.g., separating uric acid from calcium in gout, or iodine maps and virtual non-contrast images.
- Slip rings
- Rotating electrical contacts that transmit power and data to the continuously spinning gantry, enabling helical (continuous-rotation) scanning.
- Data acquisition system (DAS)
- The electronics that receive the detector signals, convert them from analog to digital, and pass the projection data to the array processor for reconstruction.
- Detector quantum efficiency (DQE)
- How efficiently detectors convert incident x-ray photons into usable signal. Higher DQE means lower noise for a given dose — better dose efficiency.
- Partial volume averaging
- An artifact where a voxel spans more than one tissue type, so its HU is an average of them — causing blurred or inaccurate values. Reduced by thinner slices.
- Metal artifact
- Streaks and dark/bright bands from high-density implants caused by beam hardening, photon starvation, and scatter. Reduced with higher kVp, metal artifact reduction (MAR) algorithms, and gantry angulation.
- Ring artifact
- Concentric circular artifact from a miscalibrated or faulty detector element in a third-generation rotate-rotate scanner; corrected by detector calibration.
- Spatial resolution
- The ability to distinguish small, closely spaced high-contrast objects (line pairs/cm). Improved by smaller FOV, thinner slices, sharp kernels, and a smaller focal spot.
- Contrast resolution
- The ability to distinguish tissues with similar attenuation (low contrast). CT excels here; improved by higher mAs (less noise), smooth kernels, and lower kVp (more iodine contrast).
- Effect of lower kVp on iodine
- Lower kVp brings the beam energy nearer iodine's k-edge (33 keV), increasing iodine attenuation/contrast and often allowing lower contrast dose — but it increases image noise unless mAs is raised.
- Field of view (FOV) and resolution
- Display/scan FOV sets how the matrix maps to anatomy. A smaller FOV spreads the same matrix over less area, giving smaller pixels and higher spatial resolution.
- Image noise in CT
- Grainy variation in CT numbers, dominated by quantum mottle (too few photons). Reduced by raising mAs, lowering kVp's effect via thicker slices, smoother kernels, or iterative reconstruction — at a dose trade-off.
- Scan FOV (SFOV) vs display FOV (DFOV)
- SFOV sets the calibration/bowtie and data collected; DFOV (≤ SFOV) selects the reconstructed area. A targeted small DFOV improves displayed spatial resolution.
- Multiphase / dynamic imaging
- Scanning the same anatomy at multiple time points after contrast (arterial, venous, delayed) to characterize lesion enhancement and washout (e.g., adrenal, renal, liver lesions).
- Oral contrast in abdominal CT
- Positive (iodinated/barium) oral contrast opacifies the GI tract to distinguish bowel from masses/abscess; neutral (water/low-HU) oral contrast distends bowel for wall assessment (enterography).
- Arterial phase timing
- Image acquisition during peak arterial enhancement (commonly ~ 20–35 s after injection start, varies by protocol/patient) to show hypervascular lesions and arteries (CTA).
- Portal venous phase
- Abdominal imaging ~ 60–70 s after injection, when the liver parenchyma and portal/hepatic veins are maximally enhanced — the workhorse phase for most abdominal CT.
- Coronary artery calcium (CAC) scoring
- A non-contrast, ECG-gated cardiac CT that quantifies calcified coronary plaque (Agatston score) to assess cardiovascular risk.
- Virtual colonography (CT colonography)
- Low-dose CT of the insufflated, cleansed colon with 2D and 3D 'fly-through' review to screen for colorectal polyps and cancer.
- Cross-sectional anatomy: liver segments
- The Couinaud system divides the liver into 8 functionally independent segments based on portal/hepatic venous supply — essential for localizing lesions on CT.
- Circle of Willis
- The arterial anastomotic ring at the base of the brain (anterior/posterior cerebral, communicating, internal carotid arteries) — a key target of head/neck CT angiography.
- Inverse square law
- Radiation intensity is inversely proportional to the square of the distance from the source; doubling distance reduces intensity to one quarter — the basis for maximizing distance for staff protection.
- Cardinal principles of protection
- Time, distance, and shielding — minimize time near the source, maximize distance (most effective), and use shielding (lead) to keep occupational and patient dose ALARA.
- Gray (Gy)
- The SI unit of absorbed dose — energy deposited per unit mass of tissue. CT dose metrics (CTDIvol, DLP) are expressed in milligray (mGy).
- Sievert (Sv)
- The SI unit of equivalent/effective dose — absorbed dose weighted for biological harm. Effective dose from CT is estimated in millisieverts (mSv).
- Photoelectric effect
- An x-ray interaction in which a photon is totally absorbed by an inner-shell electron — it builds image contrast, depends strongly on atomic number and lower energy, and contributes to patient dose.
- Compton scatter
- An x-ray photon ejects an outer-shell electron and continues deflected. Scatter degrades contrast and is the principal source of occupational dose to staff in the CT room.
- Over-ranging (z-overscanning)
- Extra rotations beyond the planned scan range in helical CT needed for reconstruction interpolation, adding dose. Reduced by adaptive (dynamic) z-collimation.
- Pediatric CT — Image Gently
- A campaign to child-size CT technique: lower kVp/mAs, weight/size-based protocols, scan only the indicated area, and avoid unnecessary multiphase scans, because children are more radiosensitive.
- Pregnancy and CT
- Verify pregnancy status before scanning; weigh benefit vs. fetal dose, shield/limit the field where feasible, and consider alternatives. Document justification — the embryo/fetus is highly radiosensitive.
- Collimation / beam width
- Restricting the x-ray beam to the detector array width reduces patient dose and scatter. Total beam width = number of detector rows × slice width.
- Dose notification / alert values
- Console thresholds (CTDIvol/DLP) set per protocol that warn the operator when a planned scan would exceed a facility limit — a dose-management safety check before scanning.
- Adverse event reporting
- Documenting and reporting scanning errors, dose events, and contrast reactions per facility policy and regulation — part of the CT quality and safety program.
- Informed consent (CT contrast)
- The patient must be told the procedure's risks, benefits, and alternatives — including potential contrast reactions — and consent before contrast administration; the technologist verifies it is documented.
- Time-out
- A pre-procedure pause to verify correct patient, correct procedure/site, and correct protocol/orders before scanning — a patient-safety standard against wrong-patient/wrong-exam errors.
- Two patient identifiers
- Confirm identity with two identifiers (e.g., full name and date of birth) — never the room/table location — before any CT exam or contrast injection.
- Normal adult vital signs
- BP ≈ 120/80 mmHg, heart rate 60–100 bpm, respiratory rate 12–20/min, SpO₂ 95–100%, temperature ≈ 37 °C (98.6 °F). Recognize deviations during monitored CT.
- Mild vs moderate vs severe contrast reaction
- Mild (limited hives, nausea, flushing) → observe; moderate (diffuse urticaria, mild bronchospasm, vomiting) → treat; severe (laryngeal edema, anaphylaxis, cardiovascular collapse) → emergency response, epinephrine, call a code.
- First action in a contrast reaction
- Stop the contrast injection immediately, maintain IV access, assess the patient (airway, breathing, circulation), call for help/physician, and follow the emergency reaction protocol.
- Power injector — single vs dual head
- A single-head injector delivers contrast only; a dual-head injector delivers contrast plus a saline flush (pushing the contrast bolus and clearing the line). Set flow rate and volume per protocol.
- Saline flush (chaser)
- Saline injected after the contrast bolus to push residual contrast from the tubing/arm veins into central circulation — tightening the bolus and reducing streak artifact at the injection site.
- Injection flow rate and gauge
- Higher flow rates (e.g., CTA) require a larger-bore, well-placed IV (e.g., 18–20 g in the antecubital). Matching flow rate to catheter/vein reduces extravasation risk.
- Lab values before contrast
- Check renal function (eGFR/creatinine) and relevant labs (e.g., per protocol). Screen for prior reactions, diabetes/metformin, asthma/allergy, and pregnancy before iodinated IV contrast.
- Infection control in CT
- Hand hygiene, gloves/PPE for body-fluid contact, aseptic technique for IV access, and cleaning the table, gantry bore, and accessories between patients (clean, disinfect, sterilize as appropriate).
- Standard precautions
- The minimum infection-control practices used for every patient regardless of diagnosis: hand hygiene, PPE as needed, respiratory hygiene, safe injection practices, and equipment cleaning.
- Brain CT slice thickness
- Thin slices (≈ 1–2.5 mm) improve detection of small intracranial lesions and reduce posterior fossa beam-hardening; the posterior fossa is often imaged thinner than the supratentorial brain.
- HRCT of the chest
- High-resolution CT uses thin slices and a sharp (high-spatial-frequency) kernel to evaluate the lung parenchyma and interstitial disease (e.g., interstitial lung disease, nodules).
- Low-dose lung cancer screening CT
- A non-contrast, low-dose chest CT used to screen high-risk patients for lung cancer, minimizing dose while detecting pulmonary nodules.
- Cross-sectional anatomy: mediastinum
- The central thoracic compartment between the lungs containing the heart, great vessels, trachea, esophagus, thymus, and lymph nodes — a key region on chest CT.
- Adrenal washout CT
- Multiphase adrenal imaging (non-contrast, portal-venous, and 15-min delayed) calculating absolute/relative washout to distinguish benign adenomas from other lesions.
- CT angiography (CTA)
- Contrast-enhanced CT timed to peak arterial enhancement with thin slices, used with MIP/3D reformations to evaluate arteries (e.g., aorta, coronaries, pulmonary, runoff).
- Appendicitis CT
- Abdomen/pelvis CT extended through the pelvis to fully include the appendix; findings include a dilated, non-filling appendix with wall thickening and periappendiceal fat stranding.
- Temporal bone CT
- A dedicated thin-slice, high-resolution, bone-kernel study of the petrous temporal bones / internal auditory canal for the ossicles, cochlea, and middle/inner ear structures.
- Gantry tilt / planning the spine
- Aligning the acquisition or reformations parallel to the intervertebral disc spaces optimizes visualization of disc spaces and spinal alignment.
- Trauma CT (pan-scan)
- Rapid contrast-enhanced CT of head, c-spine, chest, abdomen, and pelvis to survey major injuries in the unstable/multitrauma patient.
- CT-guided biopsy / drainage
- Interventional procedures using CT to guide needle placement for tissue sampling (biopsy) or to drain fluid collections/abscesses, with strict aseptic technique.
- Renal stone CT findings
- On non-contrast CT, calculi appear as high-attenuation foci; secondary signs include hydronephrosis, ureteral dilation, and perinephric stranding.
- Lung cancer staging — why include liver and adrenals
- The liver and adrenal glands are common metastatic sites for lung cancer, so the staging chest CT extends to include the upper abdomen.
- Delayed (excretory) phase urography
- Imaging ~ several minutes after injection when contrast is excreted into the collecting systems and ureters — assessing patency and filling of the urinary tract.
- Hepatocellular carcinoma enhancement
- HCC classically shows arterial-phase hyperenhancement with portal-venous/delayed washout — the reason multiphase liver CT is performed.
- Reconstruction interval (overlap)
- The spacing between reconstructed slices. Overlapping (interval < slice thickness) improves MPR/3D quality and lesion detection without re-scanning the patient.
- Volume rendering (VR)
- A 3D post-processing technique that assigns color/opacity to ranges of HU to display surfaces and depth — useful for vascular and skeletal anatomy.
- SSD (shaded surface display)
- A 3D rendering showing only the surface of structures above a chosen threshold — fast but discards internal data, largely replaced by volume rendering.
- Matrix and pixel size
- The reconstruction matrix (e.g., 512 × 512) and the FOV together set pixel size: pixel size = FOV ÷ matrix. Smaller pixels (smaller FOV or larger matrix) raise spatial resolution.
- PACS / DICOM
- PACS stores, retrieves, and distributes images; DICOM is the standard format/protocol for medical image data and metadata, enabling cross-system communication and teleradiology.
- X-ray tube: cathode and anode
- The cathode emits electrons by thermionic emission; they strike the rotating anode (target), producing x-rays. The tube must dissipate large heat loads during continuous CT scanning.
- Sequential (axial / step-and-shoot) scanning
- The table is stationary during each rotation, then steps to the next position. Used where helical motion is undesirable (e.g., some perfusion or high-resolution protocols).
- TAVR planning CT
- A gated CT of the aortic root and a CTA of the vascular access route to size the valve annulus and plan transcatheter aortic valve replacement.
- Esophagram (CT esophagography)
- CT of the esophagus, often with oral contrast, to evaluate the esophageal wall, masses, or leaks.
- Retrograde cystogram (CT)
- Contrast is instilled into the bladder via catheter, then CT is performed to evaluate bladder integrity (e.g., suspected rupture).
- Iodine k-edge
- Iodine's k-shell binding energy is ~33 keV. X-ray energies just above it are strongly absorbed by iodine, which is why lower kVp boosts iodine (vascular) contrast.
- Quality assurance / QC tests
- Routine CT QC includes CT number accuracy/calibration (water = 0 HU), noise/uniformity, slice thickness, spatial/contrast resolution, and laser/alignment checks to keep the scanner within tolerance.
- CT number accuracy (water calibration)
- Water must measure 0 ± a few HU; air must be near −1000 HU. Routine water-phantom calibration keeps Hounsfield values accurate for diagnosis.
- Motion artifact
- Blurring, doubling, or streaks from patient or physiologic motion (breathing, cardiac, peristalsis). Reduced by breath-hold, faster scans, gating, and patient immobilization/communication.
- Photon starvation artifact
- Streak artifact in highly attenuating regions (e.g., shoulders, hips) where too few photons reach the detector. Reduced by tube-current modulation, higher mA, or adaptive filtering.
