Contrast CT is X-ray computed tomography (CT) using radiocontrast. Radiocontrasts for X-ray CT are, in general, iodine-based types. This is useful to highlight structures such as blood vessels that otherwise would be difficult to delineate from their surroundings. Using contrast material can also help to obtain functional information about tissues. Often, images are taken both with and without radiocontrast. CT images are called precontrast or native-phase images before any radiocontrast has been administrated, and postcontrast after radiocontrast administration.
Bolus tracking is a technique to optimize timing of the imaging. A small bolus of radio-opaque contrast media is injected into a patient via a peripheral intravenous cannula. Depending on the vessel being imaged, the volume of contrast is tracked using a region of interest (abbreviated "R.O.I.") at a certain level and then followed by the CT scanner once it reaches this level. Images are acquired at a rate as fast as the contrast moving through the blood vessels.
"Washout" is where tissue loads radiocontrast during arterial phase, but then returns to a rather hypodense state in venous or later phases. This is a property of for example hepatocellular carcinoma as compared to the rest of the liver parenchyma.
Depending on the purpose of the investigation, there are standardized protocols for time intervals between intravenous radiocontrast administration and image acquisition, in order to visualize the dynamics of contrast enhancements in different organs and tissues. The main phases thereof are as follows:
|Phase||Time from injection||Time from bolus tracking||Targeted structures and findings|
|Non-enhanced CT (NECT)||-||-|
|Pulmonary arterial phase||6-13 sec||-|
|Pulmonary venous phase||17-24 sec||-|
|Early systemic arterial phase||15-20 sec||immediately||
|Late systemicarterial phase
Sometimes also called "arterial phase" or "early venous portal phase"
|35-40 sec||15-20 sec||
|Pancreatic phase||30 or 40 - 50 sec||20-30 sec|
|Hepatic (most accurate) or late portal phase||70-80 sec||50-60 sec||
|Nephrogenic phase||100 sec||80 sec||
|Systemic venous phase||180 sec||160 sec|
Sometimes called "wash out phase" or "equilibrium phase"
|6-15 minutes||6-15 minutes|
CT angiography is a contrast CT taken at the location and corresponding phase of the blood vessels of interest, in order to detect vascular diseases. For example, an abdominal aortic angiography is taken in the arterial phase in the abdominal level, and is useful to detect for example aortic dissection.
The following table shows the preferable volume in normal weight adults. However, dosages may need to be adjusted or even withheld in patients with risks of iodinated contrast, such as hypersensitivity reactions, contrast-induced nephropathy, effects on thyroid function or adverse drug interactions.
|300 mg/ml||350 mg/ml||370 mg/ml|
|CT of brain||95ml||80 ml||75 ml|
|CT of thorax||Overall||70 - 95 ml[notes 1]||60 - 80 ml[notes 1]||55 - 75 ml[notes 1]||Parenchymal changes of the lung can often be evaluated adequately without the use of intravenous contrast.|
|CT pulmonary angiogram||20 ml[notes 2]||17 ml[notes 2]||15 ml[notes 2]||Minimal amount when using specific low-contrast protocol.[notes 2]|
|CT of abdomen||Overall||70 ml||60 ml||55 ml|
|Liver||55 ml[notes 3]||45 ml[notes 3]||40-45 ml[notes 3]||Minimal required amount.[notes 3]|
|CT angiography||25 ml[notes 4]||20 ml[notes 4]||When using specific low-contrast protocol.[notes 4]|
The dose should be adjusted in those not having normal body weight, and in such cases the adjustment should be proportional to the lean body mass of the person. In obese patients, the Boer formula is the method of choice (at least in those with body mass index (BMI) between 35 and 40):
For men: Lean body mass = (0.407 × W) + (0.267 × H) − 19.2
For women: Lean body mass = (0.252 × W) + (0.473 × H) − 48.3
Standard doses in children:
|Exam||Concentration of iodine|
|300 mg/ml||350 mg/ml|
|Generally||2.0 ml/kg||1.7 ml/kg|
|CT of brain, neck or thorax||1.5 ml/kg||1.3 ml/kg|
Iodinated contrast agents may cause allergic reactions, contrast-induced nephropathy, hyperthyroidism and possibly metformin accumulation. However, there are no absolute contraindications to iodinated contrast, so the benefits needs to be weighted against the risks.
As with CT scans in general, the radiation dose can potentially increase the risk of radiation-induced cancer.
- 0.3–0.4 gI/kg in a 70kg individual, according to:
- Iezzi, Roberto; Larici, Anna Rita; Franchi, Paola; Marano, Riccardo; Magarelli, Nicola; Posa, Alessandro; Merlino, Biagio; Manfredi, Riccardo; Colosimo, Cesare (2017). "Tailoring protocols for chest CT applications: when and how?". Diagnostic and Interventional Radiology. 23 (6): 420–427. doi:10.5152/dir.2017.16615. ISSN 1305-3825. PMC 5669541.
- Using dual energy CTA (such as 90/150SnkVp), according to:
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- The liver generally needs an enhancement of at least 30 HU for proper evaluation according to:
- Multislice CT (3 ed.). Springer-Verlag Berlin and Heidelberg GmbH & Co. KG. 2010. ISBN 9783642069680.
- CT-angiography in a 70kg person, with 100-150 mg I/kg by using 80 kVp, mAs-compensation for constant CNR, fixed injection duration adapted to scan time, automatic bolus tracking and a saline chaser, according to:
- Webb, W. Richard; Brant, Wiliam E.; Major, Nancy M. (2014). Fundamentals of Body CT. Elsevier Health Sciences. p. 152. ISBN 9780323263580.
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