Large Patients; Scanning Large Patients; Perform Only Necessary Ct Examinations; Patient Weight And Patient Size - GE Revolution CT User Manual

Tomography scanner
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2 Large Patients

2.1 Scanning large patients

2.1.1 Perform only necessary CT examinations

Is CT the most appropriate study? This important communication between the patient's
physician and the radiologist is essential in determining the need for the CT examination.
The indications and the appropriate technique to be used should be reviewed by the radiologist
prior to every scan including the patient's number of previous scans, reasons for the scan, and
consideration of other effective modalities. In all circumstances, the expected benefits of the
scan must always exceed the overall risk.

2.1.2 Patient weight and patient size

There are multiple factors that need to be taken into consideration when prescribing a protocol
for large patients besides weight such as height, muscularity, and body habitus. It is essential to
remember that the maximum table loading weight is 500 lb (227 kg).
The patient's transverse dimension of the anatomical area being scanned is another important
factor of a larger patient exam because of system hardware imaging requirements of an air gap
and design limitations of a CT system.
The distance between the edge of gantry bore and the patient's body surface must be more
than 5 cm to prevent blockage of the reference channels. If the maximum diameter of the
patient is beyond the imaging capability of the 50 cm SFOV, the image quality can be degraded
due to partial reference channel blockage. CT scanners expect an air gap around the imaged
object and when the reference channel does not see air, this may cause degradation of image
quality. If the diameter of the patient in all directions is beyond the imaging capability of the 50
cm SFOV, the image quality may be severely degraded and may not be useful for diagnosis if
there is full reference channel blockage.

2.1.3 Properly position large patients in the gantry

Body straps may be useful with certain patients. These accessories are sometimes helpful in
making the patient more round as opposed to elliptical, and in securing the patient to prevent
contact with the gantry as they move through. Once the patient is positioned on the table, move
the patient in and out of the gantry to make sure they are able to be safely moved through the
gantry during the exam.
Properly centering the patient is essential to minimize image quality degradation. The bowtie
filter provides maximum X-ray intensity through the center and 90% less X-ray at the edge of
the SFOV. The Large Body bowtie filter (Large SFOV) should be used since it has the largest
field of unattenuated X-ray. When possible, center the anatomy of interest in the center of the
gantry to take advantage of the increased X-ray intensity.
Additionally, a correctly centered patient is essential when using automatic exposure control
techniques such as SmartmA to reduce unnecessary radiation exposure. If patients are not
properly centered when using automatic exposure control, the mA selected by the system may
be inadequate or may be greater than required compared to when the positioning is correct.
Often in order to fit a large patient safely in the gantry for imaging, a lower table position than
desired is necessary. In these situations where the patient is miscentered, the use of SmartmA
is not recommended.
134
Revolution CT User Manual
Direction 5480385-1EN, Revision 1
2 Large Patients

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