RD1.1,RD2.1-2 | Imaging Modality Foundations — Glossary

The product of tissue density and sound velocity (Z = ρv); differences in acoustic impedance at tissue interfaces cause ultrasound reflection — the origin of echogenic (bright) signals on USG.

The dark band posterior to a strongly reflective/attenuating structure on ultrasound (e.g. behind a gallstone), a key sign used to identify calculi.

Evidence-based guidelines on the most appropriate imaging study for a given clinical scenario; together with AERB referral guidance they underpin modality-selection defaults.

Atomic Energy Regulatory Board — India's statutory radiation safety regulator established under the Atomic Energy Act 1962, responsible for licensing diagnostic X-ray and CT equipment and mandating justification of ionising procedures.

As Low As Reasonably Achievable — the radiation-protection principle requiring that doses be minimised to the lowest level that achieves the diagnostic goal, and that every ionising study be justified.

A non-IgE-mediated acute hypersensitivity-like reaction to contrast media; a prior severe anaphylactoid reaction to iodinated contrast mandates premedication or an alternative study.

The high-level category of disease driving modality choice in RD2.2 — infection, tumour, trauma, or congenital/structural abnormality.

A pacemaker or implantable cardioverter-defibrillator; historically an MRI contraindication, though many newer devices are MRI-conditional and may be scanned under a defined protocol with cardiology involvement.

An acute decline in renal function within 48–72 hours of iodinated contrast administration, with risk concentrated in patients with pre-existing renal impairment (eGFR <30 highest risk); previously termed contrast-induced nephropathy.

A contrast-enhanced CT of the pulmonary arteries used to diagnose pulmonary embolism by demonstrating intraluminal filling defects.

A congenital hip abnormality imaged first by ultrasound in young infants (before femoral-head ossification) and by plain pelvic X-ray in older children — an example of patient-group-driven modality choice.

An MRI sequence that detects restricted water diffusion; it shows acute ischaemic infarction within minutes to hours, when non-contrast CT is typically still normal.

A national or international benchmark dose for a standard diagnostic procedure; facilities whose doses routinely exceed the DRL are expected to review and reduce their practice.

A composite radiation dose quantity (unit: millisievert, mSv) that accounts for the differing radiosensitivity of exposed tissues, enabling comparison of risk across modalities and procedures.

Estimated glomerular filtration rate (mL/min/1.73 m²) — the renal-function measure central to contrast screening; <30 marks the high-risk threshold for both contrast-associated AKI and nephrogenic systemic fibrosis.

A rapid bedside ultrasound examination for free intraperitoneal (and pericardial) fluid in trauma; radiation-free and especially useful in children and pregnancy as an initial assessment.

A retained iron-containing fragment (e.g. an intra-ocular metal splinter in a metal-worker) that can move under the MRI static field and cause injury; an orbital X-ray is performed before MRI when suspected.

The imaging test recommended as the initial investigation for a given clinical problem, based on diagnostic yield, availability and acceptable harm (e.g. ultrasound for biliary disease, non-contrast CT for acute intracranial haemorrhage).

A gadolinium-chelate contrast medium used in MRI to shorten T1 relaxation; it contains no iodine, is not a substitute for iodinated contrast in CT, and risks NSF in severe renal failure.

The CT attenuation unit; water = 0 HU, bone ~400–1000 HU, air −1000 HU, fat −100 to −50 HU — used to characterise tissue density on CT.

The clinical reasoning of choosing the correct imaging test for a given pathology type, body region and patient group so that the study answers the clinical question with the least harm (competency RD2.2).

Radiation intensity diminishes in proportion to the square of the distance from the source (I ∝ 1/d²); doubling distance reduces dose to one-quarter — the physical basis of distance as a radiation protection measure.

An iodine-based radio-opaque contrast medium used in CT, fluoroscopy and angiography; renally excreted, it carries risks of CA-AKI, anaphylactoid reactions, and (rarely) Jod–Basedow hyperthyroidism.

An imaging modality that deposits ionising radiation — X-ray, fluoroscopy, CT and nuclear medicine — used when its speed, sensitivity or specific information is decisive and the exposure is justified.

Iodine-induced hyperthyroidism that can be precipitated by a large iodine load (such as iodinated contrast) in a susceptible patient with autonomous thyroid tissue.

The screening principle that an ionising study must be expected to change management and have no equivalent non-ionising alternative; mandated for Indian facilities by the AERB.

The resonance frequency at which protons precess in an external magnetic field, proportional to field strength (42.58 MHz/T for hydrogen); the RF pulse must be tuned to this frequency to excite protons in MRI.

A biguanide oral hypoglycaemic; because it risks lactic acidosis if contrast-associated AKI develops, it is withheld at the time of iodinated contrast and for 48 hours when eGFR <60 or a large contrast volume is used (ESUR).

An implant certified safe for MRI ONLY under specified conditions (field strength, SAR limits, positioning, monitoring) per the manufacturer; distinct from, and not equivalent to, 'MRI-safe'.

A rare fibrosing disorder of skin, joints and internal organs associated with gadolinium exposure in patients with severe renal failure (eGFR <30); risk is higher with linear than macrocyclic gadolinium agents.

The first-line study for acute intracranial haemorrhage and acute head trauma, where fresh blood appears hyperdense (bright); it is used to exclude haemorrhage before thrombolysis in stroke.

An imaging modality that uses no ionising radiation — ultrasound and MRI — preferred wherever it answers the clinical question, particularly in children and pregnancy.

A fracture not visible on initial plain X-ray (e.g. a scaphoid fracture) that may require repeat imaging or MRI, the most sensitive modality for the associated marrow oedema.

The increased vulnerability of children to radiation-induced harm (rapidly dividing tissues and a longer lifetime for cancer to manifest), which lowers the threshold to choose non-ionising modalities and mandates dose-reduced protocols.

The structured clinical assessment performed before imaging to determine whether a patient can safely undergo a study — covering allergies, renal function, pregnancy and implanted devices (competency RD2.1).

Positron emission tomography combined with computed tomography; uses a positron-emitting tracer (typically ¹⁸F-FDG) to image metabolic activity co-registered with CT anatomy — primary tool for cancer staging and treatment response.

The dominant X-ray–tissue interaction at lower photon energies; the photon is completely absorbed by an inner-shell electron, which is ejected — responsible for the high contrast between bone and soft tissue on plain X-rays.

The property of certain crystals (e.g., PZT) to generate electrical voltage when mechanically deformed, and to deform when a voltage is applied — the basis of ultrasound transducers.

A compound labelled with a gamma-emitting or positron-emitting radionuclide administered to a patient in nuclear medicine; it localises to target tissue based on biochemical behaviour, enabling functional imaging.

The use of sedative or general anaesthesia to keep a patient (often a young child) still for studies such as MRI or CT; its own risks make it an additional reason to prefer ultrasound in children.

A radiation effect (principally carcinogenesis and hereditary effects) assumed to have no safe threshold, with probability proportional to dose — the rationale for justifying every ionising study, especially in the young and pregnant.

The time constant (T1) by which protons return their longitudinal magnetisation toward equilibrium after an RF pulse; fat has short T1 (bright on T1-weighted MRI), fluid has long T1 (dark).

The time constant (T2) describing decay of transverse magnetisation; fluid has long T2 (bright on T2-weighted MRI), solid tissues have short T2 (dark).