The operation of the bowtie filter in x-ray CT is correct if the object being scanned
is properly centered in the scanner’s field-of-view. Otherwise, the dose delivered to the
patient and image noise will deviate from optimal setting. We investigate the effect of miscen-
tering on image noise and surface dose on three commercial CT scanners. Six cylindrical phan-
toms with different size and material were scanned on each scanner. The phantoms were
positioned at 0, 2, 4 and 6 cm below the isocenter of the scanner’s field-of-view. Regression
models of surface dose and noise were produced as a function of miscentering magnitude
and phantom’s size. 480 patients were assessed using the calculated regression models to esti-
mate the influence of patient miscentering on image noise and patient surface dose in seven
imaging centers. For the 64-slice CT scanner, the maximum increase of surface dose using the
CTDI-32 phantom was 13.5%, 33.3% and 51.1% for miscenterings of 2, 4 and 6 cm, respectively.
The analysis of patients’ scout scans showed miscentering of 2.2 cm in average below the iso-
center. An average increase of 23% and 7% was observed for patient dose and image noise,
respectively. The maximum variation in patient miscentering derived from the comparison
of imaging centers using the same scanner was 1.6 cm.
مقاله : Impact of miscentering on patient dose and image noise in x-ray CT imaging Phantom and clinical studies