|Vol. 4, Issue 2, Article 2||Sanelli, P., Shetty, S. & Lev, M.|
CTA/CTP scanning protocol parameters will vary depending on the specific scanner type and software available. The following are sample protocols, reflecting the general principles of perfusion CTA/CTP scanning. Specific, up-to-date protocols for multislice scanners from various vendors can be obtained from Michael H. Lev, MD, upon request (email@example.com). Patient preparation includes removing metallic hardware, including dental and hair prosthetics, so as to minimize distortion artifacts. The protocol for performing CT perfusion, with deconvolution-based map reconstruction, employs cine scanning mode (acquisition of one-to-two images per second) with kVp 80, mAs 190, and power injection of 40-45 cc of non-ionic (preferably low osmolar) contrast (300 mg iodine/mL) at 4.0 - 7.0 cc/sec, with a 5 second delay (Fig. 6). Theoretically, for a given mAs, this nonstandard kVp setting may increase the conspicuity of contrast due to the photoelectric effect for 80 kVp photons, which are closer to the k-edge of iodine than the 120 - 140 kV photons used for routine head CT scanning (18). This is important for maximizing the signal-to-noise ratio of the tissue TDC's. One image per second is typically sufficient to produce quality maps, however two images per second will result in less image noise. The gantry angle is parallel and superior to the orbital roof, so as to minimize radiation exposure to the lens (Fig. 6). The CTP source images are transferred to a workstation for post processing.
The imaging protocol for acute stroke patients includes an initial non-contrast CT (NCCT) of the head, CT angiogram (CTA), and CT perfusion. CTA is performed from aortic arch to the vertex to visualize the extracranial and intracranial segments of the cerebral vasculature. An additional advantage of this protocol is that, when the arterial and tissue contrast concentrations are at an approximate steady state, the source images from the CTA acquisition are blood volume weighted. CT perfusion is finally performed, with approximately 2 - 4 cm of coverage per contrast bolus, depending on the precise make and model of scanner used, centered on the suspected stroke territory.
The potential nephrotoxicity of iodinated contrast can be reduced in this protocol by using low or iso-osmolar contrast, coupled with a "saline chaser" immediately following contrast injection. The latter technique requires sequential administration of contrast followed by saline, in order to "wash-out" otherwise unused contrast in both the IV line and proximal veins. This "saline chaser" also results in decreased streak artifact from high concentrations of contrast pooling in the brachiocephalic vein on the side of the contrast injection. Biphasic or multiphasic injection protocols are also useful to achieve a steady-state plasma concentration of contrast, or for other "bolus shaping".
The greatest limitation of performing CT perfusion studies is the limited volume of coverage that can be obtained during a single acquisition, particularly compared to the whole-brain coverage offered by MR perfusion. In order to expand the anatomic coverage beyond a single "slab", two techniques can be employed. The "toggle-table" technique, which allows selection of two discrete slabs, each 2 cm in thickness, to be scanned in an alternating fashion at two different table locations (19), compromising temporal resolution in favor of increased coverage, is not in widespread use. More accepted has been the use of two successive CT perfusion acquisitions, albeit at a cost of increased total contrast dose (20). At our institutions, the inferior CTP slab is placed at the top of ICA, and the superior slab is placed at the level of the A2 segments. For each series, 40-45 cc of non-ionic contrast injection is required. Post processing is performed separately at these two locations. This technique offers temporal resolution without altering the quantitative accuracy of the results. Further techniques for increasing CT perfusion coverage will continue to improve as the next generation of multi-detector (64 slice) scanners is developed. Of note, this "next generation" of scanners may obviate the need to employ some of the scan acquisition pearls listed below.
|Pearls: Head CT Scanning Protocols
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