Physics
CT (computed tomography) uses X-rays to obtain images. A heated cathode releases high-energy electrons, which in turn release their energy as X-ray radiation. X-rays pass through tissues and hit a detector on the other side.
The more dense a tissue, the more X-rays it absorbs.
- Bone: X-rays absorbed = few X-rays reaching detector: White
- Air: X-rays not absorbed = lots of X-rays reaching detector: Black
Compared to plain film, CT is able to distinguish more subtle density differences and there is no overlap of structures.
Current CT machines use ‘Spiral CT’. This consists of a single radiation source with multiple detectors which rotates around the patient, obtaining a block of data as the patient is moved through.
Axial slices: “as if looking at patient from the foot end of the bed”
The information obtained can be reconstructed by a computer to form a 3D “volume”, which can then be “re-sliced” digitally to obtain thinner slices as well as slices in different planes.
Phases of a scan refer to when the images are taken, relative to time of contrast administration.
non or pre-contrast > arterial > venous > delayed
The arterial phase comes before the venous phase, because even though contrast is given into a vein, within approximately 30 seconds, the contrast has passed through the heart and into the arterial system.
- The chest is usually imaged in the arterial phase.
- The abdomen is usually imaged in the (portal) venous phase.
- Liver lesions are usually imaged with a multiple phase scan.
In a CT chest/abdo/pelvis, the lung bases/liver may be imaged twice (overlap between chest - arterial phase, and abdomen - venous phase)
Artefacts can complicate interpretation. Examples:
- metal (usually seen as radiating bright streaks): sternotomy wires, aneurysm clips, dental fillings
- high concentration of IV contrast: arm veins, heart
- motion: minimised by asking patient to hold their breath
Dental artefact
Contrast artefact
Windowing
Tissue density is measured in Hounsfield units (HU).This is defined as Air = −1000 HU; Water = 0 HU.The HU value of a certain pixel can be ascertained by moving the cursor over it in PACS. This is useful when determining the composition of a mass, or determining whether fluid is blood.
Density of tissues on CT:
(A way to remember this: Fat floats on water, so is less dense than fluid;Soft tissue is mostly intracellular fluid with some connective tissue)
Air < Fat < Fluid < Soft tissue < Bone < Metal
- Air = −1000 HU
- Lung ≈ −500 HU (partially air, partially soft tissue)
- Fat ≈ −50 HU (slightly less dense than simple fluid)
- Water = 0 HU
- Soft tissue (& blood) ≈ +50 HU (slightly more dense than simple fluid)
- Bone ≈ +1000 HU (much more dense)
A ‘level’ and a ‘width’ is defined.For example, a window with a level of 0 HU and a width of 400 HU will have a range of −200 HU to +200 HU. Any tissue with a density of −200 HU or less will be black, and any tissue with a density of +200 HU or more will be white.
A ‘window’ can be set to look at certain tissues of interest.
A small range of tissue density is represented by a full greyscale spectrum from black to white, thus making subtle density differences within the specified range easier to see.
Examples of commonly used windows are soft tissue, lung, and bone.
Soft tissue window
A soft tissue window is used to view most organs. A soft tissue window cannot be used for lung parenchyma, as lung density (−500 HU) is outside range and will appear completely black.
Level: +50 HU; Width: 350 HU (Range: −125 to +225)
Soft tissue window on a chest CT
Lung window
A lung window is used to view lung parenchyma.Lung parenchyma (−500 HU) would be within range, appearing grey. Air pockets (−1000 HU) around the lung, such as pneumothorax or bullae, would appear black, thus allowing clear differentiation.
Level: −200 HU; Width: 2000 HU (Range: −1200 to +800)
Lung window on a chest CT
Bone window
A bone window is used to view bone detail.There is good differentiation within the range of high densities found in the bony cortex and medulla.
Level: 300 HU; Width: 2000 HU (Range: −700 to +1300)
Bone tissue window on a chest CT
Radiation
In the UK, radiation exposure for medical purposes (diagnostic/treatment) is regulated under Ionising Radiation (Medical Exposure) Regulations 2000 (IRMER 2000). It provides guidance for three roles:
- Referrer (clinician) – provide adequate clinical details
- Practitioner (radiologist) – ensure scan is justified
- Operator (radiographer) – minimise amount of radiation
The ‘millisievert’ (mSv) is a unit used to measure radiation dose.The annual background radiation in the UK is 2.7 mSv.
Modality
Dose
Equivalent background radiation
Ref: Fred A. Mettler, Jr., et al., "Effective Doses in Radiology and Diagnostic Nuclear Medicine: A Catalog," Radiology Vol. 248, No. 1, pp. 254-263, July 2008.
Risks of radiation exposure:
- Deterministic - Dose-related effects: affects fast dividing cells (GI tract and bone marrow). E.g. radiotherapy treatment
- Stochastic - Random effects: causes changes in DNA (teratogenic). E.g diagnostic tests
Contrast
Iodine-based IV contrast medium is used for most CTs.
Exceptions where contrast is usually not used:
- CT KUBs (looking for renal stones)
- CT heads (unless a mass lesion is suspected)
- Poor renal function (eGFR <30) - non-contrast CT or alternative modalities used
Risks of contrast administration:
- Contrast-induced nephropathy - Creatinine increasing 25% within 3 days, with no other apparent reason. Usually self-limiting but can cause complications of kidney disease. Incidence of ~2% in patients without risk factors (e.g. diabetes)
- Anaphylaxis (immediate, within 1 hour)
- Allergy (delayed, up to 7 days)
Contraindications
No absolute contraindications to CT, main concern is over contrast and radiation dose.
- Patients with eGFR <60 but >30 can have contrast if they are given prehydration (oral and IV fluids).
- Pregnant women and children can have a CT if indicated (although alternatives would be considered first).
Other uses of CT include high-resolution CT (HRCT) protocol for imaging interstitial lung disease, virtual 3D CT colonogram as an alternative to invasive colonoscopy, and CT-guided biopsies.
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