Christoph R Becker MD
University Hospital Munich
Department of Clinical Radiology
In recent years, multislice CT has evolved to a rapid and extensively used imaging tool for a wide range of different clinical applications such as tumour staging, preventive care screening and CT angiography of any vascular territory. CT screening is usually performed without any contrast media, and tumour staging requires only slow contrast administration for homogenous and simultaneous enhancement of vessels and parenchymal organs.
However, for CT angiography, special intravenous contrast administration schemes are required, aiming at achieving the brightest possible enhancement of the vascular territory in the scan range. In general, faster multidetector CT (MDCT) scanners with higher numbers of detector rows and shorter gantry rotation times have a short acquisition time and allow for more compact administration of contrast media bolus, therefore improving the enhancement and visibility of even the smallest arteries and side branches (see Figure 1). For example, in 4-,16- and 64-detector-row CT scanners, flow rates of 3, 4 and 5ml/s are typically used, respectively. Conversely, as the scan time in CT scanners with higher numbers of detector rows decreases, the amount of contrast media may also be reduced.
The development of higher-number detector row CT scanners may further be improved by the use of highly concentrated contrast media (400 or 370mg iodine per ml). However, these high-contrast concentrations usually go along with high viscosities, which makes it difficult to administer with high-flow rates. High viscosity may partially be overcome by warming up the contrast media to body temperature. Consequently, contrast injectors used for CT angiography with high-number-detector-row CT scanners should have a warming device in order to maintain the temperature of the already prewarmed contrast media.
The use of a saline chaser after the administration of the contrast bolus may offer a number of advantages. First, as it helps to keep the contrast bolus compact, it improves the enhancement of the vessels for CT angiography. In addition, a saline chaser bolus pushes highly concentrated contrast media with high viscosity further through the peripheral vein. Flushing the peripheral veins also reduces artefacts that may occur from dense contrast media that remains in the subclavian vein while scanning. In general, parts of the contrast media may be replaced by saline with same enhancement as the initial contrast protocol without saline. On one hand, replacing contrast agent by saline saves money; on the other, reduced amounts of contrast media also reduce the risk of contrast-induced nephropathy. As lack of hydration is one of the key elements for contrast-induced nephropathy, we generally administer the entire volume of the saline syringe to every patient for hydration.
Body weight included in calculations
Newer schemes for contrast application calculate the amount of contrast media according to the body weight of the individual patient. This contributes to the fact that the blood volume in which the contrast media mixes is larger in patients with higher body weight. In addition, image noise increases with the patient’s diameter. To a certain extent, improved enhancement in heavier patients may compensate for the higher image noise by improving signal-to-noise ratio. We currently calculate 0.5g of iodine per kilogram of body weight. For CT angiography, the calculated volume may then be administered within a given period of time that is longer than the actual scan time (eg, 20s injection time in a 10s scan time). Consequently, the contrast injection rates increase with higher body weights. Highly concentrated contrast media thereby helps to keep the flow rates in obese patients within reasonable limits. Furthermore, the calculated amount can be divided by the serum creatinine to contribute to the reduced renal function and the increased risk of contrast-induced nephropathy.
Bolus tracking software
A small amount of contrast media (20ml) may be used as a test bolus, allowing determination of the arrival time of the contrast media in the larger vessels and to get an idea of the resulting enhancement of the main contrast application. Bolus tracking software automatically detects the arrival of the main contrast bolus and then initiates the CT scan. As this feature works reliably, it makes the test bolus administration for a CT angiography study redundant. For automated bolus tracking, a region of interest is placed in the ascending aorta and repeated scanning is performed, until a certain predefined density is reached by the arrival of the contrast bolus. A certain time delay should be added to allow for maximum and complete enhancement of the vascular territory that is being scanned.
In particular, in cardiac CT angiography, formerly performed with 4-detector-row CT and scan times of 40s, contrast administration needs to be maintained for the entire period of time of scanning, leading to a moderate enhancement of the left ventricle but at the same time very bright enhancement in the right ventricle. This may typically go along with artefacts in the right atrium caused by incomplete mixture of contrast media with the unenhanced blood from the inferior vena cava. Commonly, these artefacts may lead to streaks that infer with the right coronary arteries located next to the right atrium in the right atrioventricular groove.
Cardiac CT angiography should be performed exclusively with 64-detector row CT scanners because of the high spatial and temporal resolution of these scanners. Meanwhile, a breath-holding time of only 10s is necessary to acquire the entire heart by a 64-detector-row CT. The injection of a compact contrast bolus followed by a saline chaser, combined with adequate timing of the CT scanning, leads to a special effect where all the contrast is enhancing the left ventricle and coronary arteries and at the same time the saline washes out the contrast media from the right ventricle. This contrast enhancement scheme may be called CT laevo-cardiogram and allows for improved visibility of the right coronary artery while only minimal amounts of contrast media are necessary (see Figure 2).
However, as a disadvantage of this approach, the myocardial septum and the right ventricular myocardium are inaccessible without any contrast in the right ventricle during scanning. Therefore, instead of using a pure saline chaser, the second phase of the administration may consist of contrast media diluted by saline in a 1:1 ratio. As a result, the right ventricle may have some, but not too much, enhancement while the left ventricle and the coronary arteries are fully enhanced.
The latest scanning protocols cover not only the heart but also the entire chest within their scan range. In clinical terms, such protocols may be used for ruling out pulmonary embolism, aortic dissection or coronary artery disease at the same time. Conversely, this means that the enhancement has to be maintained for all the vascular territories (eg, pulmonary arteries, aorta and coronary arteries) at the same time while scanning. To get an idea of how long the contrast bolus needs to be maintained for this protocol, one should consider that the contrast media, once it arrives in the pulmonary artery, needs to travel through the lung capillary in order to arrive in the left heart and aorta. This transit time is highly variable and, in our experience, may range between 4–12s from patient to patient. Contrast injection time needs to be at least as long as the transition and the scan time to ensure simultaneous enhancement of both left and right vascular territories at the same time. Bolus tracking may be used to detect the arrival of the contrast bolus in the last territory (ie, the aorta). With the 64-detector-row CT and 15s scan time of the entire chest with ECG gating, the contrast media bolus needs to be maintained for about 30s to ensure a fully enhanced scan of all vascular territories simultaneously.
Dual-head injectors are mandatory for CT angiography and cardiac CT in particular, which are performed with high-number-detector-row CT scanners. These injectors may help to ensure high-quality CT scans with high contrast in the vascular territory of interest and less artefacts in the superior vena cava and right heart. High-quality CT angiography scans with bright enhancement are likely to improve the diagnosis of stenosis as well as the detection of atherosclerotic plaques in the coronary arteries. Pressure limits and devices to detect extra- or paravasation of contrast media in the subcutaneous or intramuscular space are also mandatory or warranted to prevent serious complications such as a compartment syndrome.