2.2 CT Perfusion
NOTICE
Please refer to the Safety section for important safety information regarding the
use of the equipment and software on this system.
NOTE:
Refer to the accompanying documents.
See the Revolution CT Technical Reference Manual, Dose and Performance chapter
for information on dose measurements and calculations.
2.2.1 Purpose of CT Perfusion
CT perfusion studies are used to assess delivery and perfusion of blood in an organ and/or its
tissues. Such studies are valuable for evaluating blood supply to neoplastic and non-neoplastic
tissue (including normal and ischemic tissue). In particular, CT perfusion imaging allows the
evaluation of cerebral ischemia or of the extent of angiogenesis associated with a tumor. CT
perfusion should be performed only for a valid medical reason and with the minimum radiation
dose necessary to achieve an adequate exam. Use of perfusion scans in children should be
particularly reviewed for clinical impact and justified. Pediatric patients are more radiosensitive
than adults and have a longer post-exam life expectancy, so particular attention should be paid
to displayed CTDIvol when modifying protocols.
CT perfusion imaging relies on the linear relationship between CT attenuation, expressed in
Hounsfield Unit (HU) and represented in a particular pixel of an image, versus the amount of
iodinated contrast material perfusing the corresponding region of tissue attenuating the X-rays.
Dynamic CT scanning enables the calculation of perfusion parameter maps, e.g., anatomic
images where the pixel value represents mean transit time, blood flow, blood volume, and
permeability maps depending upon the post-processing model used.
Scan technique parameters (e.g., kV, mAs) for CT perfusion studies should be set at values
lower than those used for routine diagnostic scanning of the same anatomical area. Perfusion
imaging involves visualization of temporal changes in iodine enhancement, rather than
resolution of small or subtle anatomical detail. The post-scan software processing of the data is
relatively insensitive to the increased noise levels; hence perfusion scans do not require use of
the same radiation levels. In general, lower kV improves visualization of iodine contrast and
consequently allows use of lower radiation doses. Lower kV settings are therefore
recommended to be used as long as sufficient image quality for perfusion post-processing can
be obtained. Body perfusion imaging of obese patients, for example, may be an application that
requires use of higher kV values. Users should carefully review the manufacturer’s reference
perfusion protocols, which reflect the recommended kV, mA, and scan time for a typical
perfusion acquisition. Additional guidance may be obtained from professional societies,
regulatory agencies, educational textbooks, or peerreviewed literature. The AAPM provides a
set of reasonable scan protocols for CT brain perfusion imaging that is freely available via its
public webpage. See the recommended readings.
Because CT perfusion requires specialized post-processing software, a CT perfusion acquisition
should not be performed unless this software is readily available to the institution. All users
should be trained in both CT perfusion acquisitions and post-processing and should follow
professional society perfusion practice guidelines. Before any changes are made to the
manufacturer’s reference protocols, both a radiologist and medical physicist familiar with CT
perfusion should be consulted. Changes in protocol and the reason for the changes should be
Revolution CT User Manual
Direction 5480385-1EN, Revision 1
220
2 Scan Theory
