医学文章阅读——Intravenous Contrast Medium Enhancement and Timing of CT Data Acquisition
Intravenous enhancement is used routinely for thoracic CT examinations, most frequently for lung cancer staging, CT pulmonary angiography (CTPA), CT coronary angiography (CTCA) and aortic evaluation. Intravenous enhancement in general is influenced by several factors. These include
• Patient factors, such as body size (as measured by body mass index and body surface area), and cardiac output;
• Contrast medium factors, including the iodine delivery rate (itself a product of iodine concentration, injected contrast volume and the rate of injection);
• Timing of acquisition, depending on whether automated bolus triggering, test bolus or a set delay is used to initiate the CT data acquisition; and
• Changes in respiration, e.g. suboptimal pulmonary artery opacification in CTPA on deep inspiration, due to a variety of suggested mechanisms.
With single-detector CT, a volume of 100 mL of 150 mg mL−1 of iodine injected at a rate of 2.5 mL s−1 after a 25-s delay was recommended for general thoracic work, while 120–140 mL of 240–300 mg mL−1 of iodine injected at a rate of 3–4 mL s–1, with either a fixed delay or the use of automated triggering mechanisms, was recommended for CTPA. However, it has been necessary to redesign contrast administration protocols and the timing of acquisition, caused by: (A) the reduced acquisition time brought about by MDCT; (B) newer technology, such as dual-energy CT; (C) the need to reduce contrast dose to minimise potential nephrotoxicity; and (D) the increasing feasibility of 'triple-rule-out' CT to provide simultaneous evaluation of the coronary arteries, pulmonary arteries and aorta, as well as other intrathoracic pathological features in patients presenting with acute chest pain.
In general, the faster acquisition times of MDCT require a higher iodine delivery rate to achieve speedier peak arterial contrast enhancement. If contrast volume is to be reduced, a higher rate can be achieved by a faster injection rate and a higher concentration of contrast medium. In addition, biphasic injection protocols are now preferred. Single-bolus (i.e. monophasic) contrast administration can cause thoracic MDCT acquisitions to suffer from streak and beam-hardening artefacts, due to the dense contrast medium within the brachiocephalic veins during CT data acquisition. To overcome this, biphasic injection protocols are now the norm, using dual-headed power injectors to deliver both contrast medium and a saline chaser to dilute the contrast density in the peripheral veins, thus overcoming the artefacts, as well as providing a more homogeneous enhancement profile. For 64-slice thoracic MDCT acquisitions, typical injection parameters are 60–120 mL of 320–400 mg mL−1 of iodine injected at 3.5–5 mL s−1, followed by 20–40 mL of normal saline injected at the same rate.
For triple-rule-out studies, variations in the number of phases, timing, volume and composition of the chaser (e.g. using a mixture of 50 : 50 contrast material and saline) are employed. For example, a triphasic protocol comprising an initial injection of undiluted contrast medium, followed by a second phase with a mixture of contrast media and saline and, finally, a third pure saline flush may be used.
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