Imaging description

The attenuation of blood depends on the hematocrit but is usually 20–35 HU. Subacute blood has a higher attenuation (50–80 HU) than flowing blood. This is because of a reduction of water content, which raises the concentration of hemoglobin. This property makes it possible to identify a vascular process such as an aortic rupture (Figure 60.1), aortic injury (Figure 60.2), acute thrombus (Figure 60.3), intramural hematoma or instability of aneurysm on non-contrast CT, by the “hyperattenuating sign.”

Hyperattenuating sign can be unmasked by using narrow windows and comparing diseased vessel to normal vessel.

Importance

Understanding the value of non-contrast CT in vascular imaging is important because sometimes iodinated contrast cannot be administered.

Typical clinical scenario

Unenhanced CT is almost routinely obtained in vascular imaging. This is to prevent mistaking inherently high-attenuation material for iodinated contrast (Figure 60.4). Additionally, after the administration of iodinated contrast the subacute hematoma in the vessel wall or thrombus becomes less apparent.

In certain circumstances iodinated contrast cannot be administered, such as impaired renal function, allergy to iodinated contrast or difficult venous access. The vascular imager, in these situations, should not write off the study because no contrast is in the arteries, as much information can still be gleaned.

Unenhanced CT may be obtained for a suspected nonvascular medical emergency such as renal stone or pancreatitis. It is important to look for an acute vascular process, which is in the differential diagnosis for renal colic and back pain.

Differential diagnosis

Aortic rupture, intramural hematoma, unstable thrombus, calcification, and periaortic hypercellular tumor.

Teaching point

Sometimes only non-contrast CT is available. The imagers should be aware that valuable information about the acuity of a vascular process can still be obtained without iodinated contrast.