# Contrast CT

Contrast CT, or contrast-enhanced computed tomography (CECT), is X-ray computed tomography (CT) using radiocontrast. Radiocontrasts for X-ray CT are generally 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 administered, and postcontrast after radiocontrast administration.

## Bolus tracking

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.
This method of imaging is used primarily to produce images of arteries, such as the aorta, pulmonary artery, cerebral, carotid and hepatic arteries.

## Washout

"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.

## Phases

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) | - | - | Calcifications Fat in tumors such as in adrenocortical adenomas Fat-stranding as seen in inflammation such as appendicitis, diverticulitis and omental infarction |
| Pulmonary arterial phase | 6-13 sec | - | Pulmonary embolism (can use bolus tracking in pulmonary trunk + 6 seconds) |
| Pulmonary venous phase | 17-24 sec | - | |
| Early systemic arterial phase | 15-20 sec | immediately | Arteries, without enhancement of organs and other soft tissues. |
| Late systemicarterial phase Sometimes also called "arterial phase" or "early venous portal phase" | 35-40 sec | 15-20 sec | All structures that get their blood supply from the arteries have optimal enhancement. Some enhancement of the portal vein |
| Pancreatic phase | 30 or 40 - 50 sec | 20-30 sec | Pancreatic cancers become hypodense compared to the parenchyma. |
| Hepatic (most accurate) or late portal phase | 70-80 sec | 50-60 sec | Liver parenchyma enhances through portal vein supply, normally with some enhancement of the hepatic veins. |
| Nephrogenic phase | 100 sec | 80 sec | All of the renal parenchyma enhances, including the medulla, allowing detection of small renal cell carcinomas |
| Systemic venous phase | 180 sec | 160 sec | Detect venous thrombosis |
| Delayed phase Sometimes called "wash out phase" or "equilibrium phase" | 6-15 minutes | 6-15 minutes | Disappearance of contrast in all abdominal structures except for tissue with fibrosis, which appears more radiodense. |

## Angiography

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.

## Amount
### Adults

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.
Sufficient volume for normal weight adults :
| Exam | Exam | Iodine concentration | Iodine concentration | Iodine concentration | Comments |
| Exam | Exam | 300 mg/ml | 350 mg/ml | 370 mg/ml | Comments |
| CT of brain | CT of brain | 95ml | 80 ml | 75 ml | |
| CT of thorax | Overall | 70 - 95 ml | 60 - 80 ml | 55 - 75 ml | Parenchymal changes of the lung can often be evaluated adequately without the use of intravenous contrast. |
| CT of thorax | CT pulmonary angiogram | 20 ml | 17 ml | 15 ml | Minimal amount when using specific low-contrast protocol. |
| CT of abdomen | Overall | 70 ml | 60 ml | 55 ml | |
| CT of abdomen | Liver | 55 ml | 45 ml | 40-45 ml | Minimal required amount. |
| CT angiography | CT angiography | 25 ml | 20 ml | 20 ml | When using specific low-contrast protocol. |
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

### Children

Standard doses in children:
| Exam | Concentration of iodine | Concentration of iodine |
| Exam | 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 |

## Adverse effects

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.
The injection of iodinated contrast agents may sometimes lead to its extravasation.