Comprehensive CT & PET/CT Revision Flashcards

Vocabulary-style flashcards covering key CT, PET/CT, and radiotherapy concepts, artefacts, protocols, pathologies, physics, contrast media, and dose considerations drawn from the lecture notes.

Pixel

Two-dimensional picture element that represents a single point in an image matrix.

Voxel

Three-dimensional volume element; each voxel has a linear attenuation coefficient used to create CT images.

Hounsfield Unit (HU)

On this scale, water is assigned a value of 00 HU, serving as a universal reference point. Air is set at −1000−1000 HU, representing the lowest attenuation. Denser tissues, such as bone, have positive HU values (e.g., +1000+1000 HU or higher), while less dense tissues or fat have negative values.

Bow-tie filter

Beam-shaping filter in CT that attenuates more in the periphery to equalise detector exposure and lower patient dose.

Pre-patient collimator

Adjustable collimator that shapes the x-ray beam before it enters the patient, helping control slice thickness and dose.

Post-patient collimator

Collimator located after the patient that removes scatter before photons reach the detectors, improving image quality.

Adaptive section collimation

Dynamic collimation that narrows the beam at the start and end of a helical scan, reducing unnecessary edge dose.

Over-beaming

Irradiation that extends beyond active detector width during helical scans, increasing patient dose without image benefit.

Over-ranging

Extra beam rotations before and after a planned volume that extend scan length beyond imaged anatomy, raising dose.

Scan Field of View (SFOV)

Maximum circular area scanned by the CT system; determines which detectors are active.

Display Field of View (DFOV)

Portion of the SFOV reconstructed for display; affects pixel size when matrix is fixed.

Interpolation (in CT)

Mathematical estimation used to convert helical data into evenly spaced axial slices.

Pitch (SSCT)

Table travel per rotation divided by slice thickness; >1 reduces dose but lowers image quality, <1 increases overlap.

Pitch (MSCT)

Table travel per rotation divided by (number of detector rows × slice thickness).

Reconstruction kernel

Mathematical filter applied during image reconstruction to emphasise either spatial resolution (sharp) or noise reduction (smooth).

Window Width (WW)

Range of CT numbers displayed as grey levels; wider WW shows more densities but less contrast.

Window Level (WL)

Mid-point of the grey scale range; sets the centre HU around which the WW is applied.

Spatial resolution (CT)

Ability of the scanner to depict small, high-contrast objects separately from each other.

Beam hardening artefact

Dark streaks or cupping produced when low-energy photons are preferentially absorbed, making the beam ‘harder’.}

Partial-volume averaging

Blurring that occurs when a voxel contains multiple tissue types, averaging their HUs.

Photon starvation artefact

Streaking in high-attenuation areas caused by insufficient x-ray photons reaching the detectors.

Undersampling artefact

Streaks or misregistration created when too few projections are acquired per rotation.

Metal artefact

Severe streaks generated by very high-density objects (e.g., prostheses) due to beam hardening and photon starvation.

Ring artefact

Concentric rings on reconstructed images caused by a mis-calibrated or defective detector element.

Cone-beam artefact

Streaks and shading at image periphery in MDCT produced by divergence of the wide cone beam.

Windmill artefact

Alternating dark-light streaks from high-pitch helical scans when interpolation across detector rows is inaccurate.

Stochastic effect (radiation)

Probabilistic biological effect with no dose threshold; probability—not severity—increases with dose (e.g., cancer).

Deterministic effect (radiation)

Biological effect with threshold dose; severity increases with dose once threshold exceeded (e.g., skin erythema).

Computed Tomography Dose Index (CTDI)

Standardised output measure representing absorbed dose per slice for a specific protocol.

Dose Length Product (DLP)

CTDI multiplied by scan length; approximates total energy imparted to the patient.

Effective dose

Weighted sum of organ doses expressing whole-body risk in millisieverts (mSv).

Automatic Tube Current Modulation (ATCM)

System that adjusts tube mA in real time according to patient size or attenuation to optimise dose.

Adaptive beam collimation

Technique that reduces tube current or closes collimators at scan extremities to minimise over-ranging dose.

CT fluoroscopy

Real-time CT imaging that provides continuous images for interventional guidance at high temporal resolution.

Dual-source CT

Scanner with two x-ray tubes and detector arrays offset 90°, enabling faster rotation and dual-energy acquisitions.

Dual Source Single Energy (DSSE)

Both tubes operate at the same kVp to double temporal resolution or widen coverage without dual-energy data.

Dual Source Dual Energy (DSDE)

Tubes run at different kVp, producing simultaneous high- and low-kV datasets for material separation.

Virtual non-contrast image

Reconstructed dataset created by subtracting iodine from a contrast scan, mimicking an unenhanced study.

Pitch versus dose relationship

Increasing pitch lowers patient dose but reduces image quality; decreasing pitch does the opposite.

CT Simulator (CT Sim)

Large-bore CT system used in radiotherapy planning to acquire images in treatment position with flat couch and lasers.

Positive contrast medium

High-attenuation agent (e.g., iodinated, barium) that appears hyperdense on CT.

Negative contrast medium

Low-attenuation agent (e.g., air, CO₂) that appears hypodense on CT.

Neutral contrast medium

Agent with density similar to water (~30 HU), providing bowel distension without high attenuation.

High-osmolality contrast media (HOCM)

Ionic agents 5–8 × plasma osmolality; higher risk of adverse reactions, mainly used orally.

Low-osmolality contrast media (LOCM)

Non-ionic agents <3 × plasma osmolality; preferred for IV use due to improved safety.

Ionic contrast agent

Dissociates into charged particles in solution; higher osmolality and neurotoxicity.

Non-ionic contrast agent

Does not dissociate; lower osmolality, better tolerability, majority of modern IV iodinated media.

Iodine concentration (contrast)

Amount of iodine (mg/mL); higher concentration increases attenuation and enhancement.

Viscosity (contrast)

Resistance to flow; increases with iodine concentration and decreases when warmed to 37 °C.

Vicarious contrast media excretion (VCME)

Alternative elimination via hepatobiliary route when renal function is impaired, producing dense bile.

Intrathecal contrast administration

Injection of contrast into the subarachnoid space for myelography; only specific low-toxicity agents allowed.

Intra-articular contrast administration

Contrast injected into a joint (arthrography) to delineate cartilage, ligaments, or joint pathology.

Saline chaser

Post-contrast saline bolus that clears contrast from tubing, optimises bolus shape, and reduces required iodine dose.

Arterial phase (CT)

~20-35 s post-injection; maximal arterial opacification before venous return, ideal for angiography.

Portal venous phase

~60-70 s post-injection; peak enhancement of portal vein and liver parenchyma, routine for abdomen.

Delayed (equilibrium) phase

3–10 min post-injection; evaluates contrast wash-out or retention in lesions and urinary tract opacification.

Time to Peak (TTP)

Interval between injection start and maximum contrast concentration in a tissue or vessel.

Peak Enhancement (PE)

Highest HU value reached by contrast within a region during dynamic scanning.

Bolus tracking

Automatic triggering of scan when contrast reaches a preset HU threshold in a target vessel.

Blood–brain barrier (BBB)

Physiologic barrier that prevents most contrast agents from entering brain parenchyma unless disrupted.

Contrast extravasation

Accidental leakage of contrast into soft tissues due to catheter displacement, causing pain and swelling.

Intracranial

Located within the skull, encompassing brain, meninges, and vascular structures.

Extra-axial

Outside brain parenchyma; includes meninges, CSF spaces, skull, and dura.

Subarachnoid space

CSF-filled space between arachnoid and pia mater; appears hypodense on CT unless filled with blood.

Grey-white matter differentiation

Normal CT contrast difference where cortical grey matter is slightly denser than medullary white matter.

Epidural haemorrhage (EDH)

Biconvex hyperdense bleed between skull and dura, usually arterial, does not cross sutures.

Subdural haemorrhage (SDH)

Crescentic bleed between dura and arachnoid; crosses sutures, density changes with age of bleed.

Subarachnoid haemorrhage (SAH)

Acute hyperdense blood within CSF spaces and cisterns, commonly from ruptured aneurysm.

Intraparenchymal haemorrhage (IPH)

Bleeding within brain tissue, often hyperdense with surrounding oedema and potential mass effect.

Intraventricular haemorrhage (IVH)

Blood within the ventricular system, often secondary to SAH or IPH; hyperdense layering in horns.

Ischaemic stroke

Brain tissue infarction from arterial occlusion; early CT shows loss of grey-white differentiation and hypodensity.

Hyperdense vessel sign

Early CT indication of acute arterial thrombosis visualised as a high-density intraluminal clot.

Meningioma

Extra-axial, usually benign tumour arising from meninges; hyperdense, may calcify and avidly enhance.

Glioblastoma multiforme (GBM)

Most aggressive astrocytic tumour; irregular peripheral enhancement with central necrosis.

Brain metastasis

Secondary malignant lesion; often multiple, peripheral, with vasogenic oedema and ring enhancement.

Hydrocephalus

Abnormal accumulation of CSF causing ventricular dilation; may be communicating or obstructive.

Circle of Willis

Arterial ring at brain base providing collateral flow between cerebral circulations.

Middle cerebral artery territory

Lateral cerebral hemispheres supplied by MCA; most common site of ischaemic stroke.

Ground-glass opacity (lung)

Hazy increased attenuation preserving bronchial and vascular margins; non-specific, common in COVID-19.

Crazy-paving pattern

Ground-glass opacity with superimposed interlobular septal thickening, seen in COVID pneumonia and others.

Traction bronchiectasis

Irreversible bronchial dilation caused by surrounding fibrotic pull, evident on HRCT.

Pulmonary embolism

Intraluminal filling defect within pulmonary artery branches, detected on CTPA.

Dual-energy CT iodine map

Colour overlay depicting iodine distribution, highlighting perfusion defects or enhancing lesions.

Thoracic aortic aneurysm

Permanent dilation ≥50 % above normal diameter; risk of rupture increases with size.

Aortic dissection

Intimal tear creating true and false lumens separated by an intimal flap; visualised with contrast CT.

Pseudoaneurysm

Contained arterial rupture bounded by adventitia or surrounding tissue, not all vessel wall layers.

High-resolution CT (HRCT)

Thin-slice, high-spatial-frequency lung imaging protocol for interstitial disease evaluation.

Honeycombing

Clustered air-filled cystic spaces (3–10 mm) representing end-stage pulmonary fibrosis.

Air trapping

Mosaic areas of low attenuation on expiratory CT, indicating small-airway obstruction.

Asbestosis

Interstitial fibrosis from asbestos exposure; CT shows pleural plaques, basal fibrosis, and honeycombing.

Sarcoidosis

Granulomatous disease; CT shows bilateral hilar lymphadenopathy and perilymphatic micronodules.

Idiopathic Pulmonary Fibrosis (IPF)

Chronic progressive fibrosing interstitial pneumonia with basal-peripheral honeycombing and reticulation.

Bullae

Air-filled spaces >1 cm resulting from emphysematous destruction of lung parenchyma.

CT pulmonary angiogram (CTPA)

Contrast CT timed to peak pulmonary artery enhancement for diagnosing PE.

PET radiopharmaceutical

Positron-emitting radionuclide linked to a biologically active molecule for functional imaging.

18F-FDG

Fluorine-18–labelled glucose analogue taken up by metabolically active tissues for PET imaging.

Standardised Uptake Value (SUV)

Semi-quantitative PET metric: tissue activity concentration divided by injected activity normalised to body size.

Attenuation correction (PET/CT)

Use of CT-derived μ-map to compensate for photon loss, producing quantitatively accurate PET images.

Co-incidence detection

PET technique that registers paired 511 keV photons arriving simultaneously to locate annihilation events.

Brown fat uptake (PET)

Physiological FDG avidity in metabolically active adipose tissue, often minimised by patient warming.