The influence of patient centering on CT dose and image noise

Abstract

Although x-ray intensity shaping filters (bowtie filters) have been used since the introduction of some of the earliest CT scanner models, the clinical implications on dose and noise are not well understood. To achieve the intended dose and noise advantage requires the patient to be centered in the scan field of view. In this study we explore the implications of patient centering in clinical practice. We scanned various size and shape phantoms on a GE LightSpeed VCT scanner using each available source filter with the phantom centers positioned at 0, 3, and 6 cm below the center of rotation (isocenter). Surface doses were measured along with image noise over a large image region. Regression models of surface dose and noise were generated as a function of phantom size and centering error. Methods were also developed to determine the amount of miscentering using a scout scan projection radiograph (SPR). These models were then used to retrospectively evaluate 273 adult body patients for clinical implications. When miscentered by 3 and 6 cm, the surface dose on a 32 cm CTDI phantom increased by 18% and 41% while image noise also increased by 6% and 22%. The retrospective analysis of adult body scout SPR scans shows that 46% of patients were miscentered in elevation by 20-60 mm with a mean position 23 mm below the center of rotation (isocenter). The analysis indicated a surface dose penalty of up to 140% with a mean dose penalty of 33% assuming that tube current is increased to compensate for the increased noise due to miscentering. Clinical image quality and dose efficiency can be improved on scanners with bowtie filters if care is exercised when positioning patients. Automatically providing patient specific centering and scan parameter selection information can help the technologist improve workflow, achieve more consistent image quality and reduce patient dose.

Automated Calculation of Water‐equivalent Diameter (DW) Based on AAPM Task Group 220

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Implications of Patient Centring on Organ Dose in Computed Tomography

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Impact of miscentering on patient dose and image noise in x-ray CT imaging: Phantom and clinical studies

“…If the object is miscentered as shown in Fig. 2, it would be exposed to more surface dose in the region that goes toward the less attenuating part of the bowtie filter and the noise would increase in the region that moves into the more attenuating part of the bowtie filter [23,24]. This is especially important for anterior and posterior organs because anterior organs receive a higher dose when a supine patient's position is under the isocenter whereas posterior organs receive Figure 1 Photograph of typical Teflon made bowtie filter which is narrow at center and thick at the edges.…”