This study was approved by the regional ethical board.
Written consent was obtained from all patients included in the study.
With a third-generation 192-section dual-source CT scanner,
we prospectively obtained two data sets each for 25 patients at 30% and 70% tube loads by proportionately splitting the tube load for a standard full-dose abdominal protocol between the two sources.
The scan parameters are shown in Table 1.
The patient population consisted of 13 females and 12 males,
age range 53 to 92 years (mean:71.6,
SD:10.1) with a BMI(kg/m2) ranging from 17.3 to 26.2 (mean 22.8; SD 2.14).
The CTDIvol ranged from 4.35 to 8.26 mGy (mean 6.41,
SD 1.17) and the DLP from 193.7 to 384.7 mGy*cm (mean 303.1,
SD 61.84).
The size-specific dose estimate (SSDE) was calculated and ranged from 6.18 to 12.16 mGy (mean 8.68,
SD 1.34).
Qualitative assessment
Five image quality criteria [5] were assessed independently by four radiologists.
The criteria used were as follows:
C1: Visually sharp reproduction of the liver parenchyma
C2: Visually sharp reproduction of the pancreas contour
C3: Visually sharp reproduction of the contours of the kidneys & proximal ureters
C4: Reproduction of contours of lymph nodes < 15mm in diameter
C5: Overall image quality for diagnostic purposes.
The images were displayed on PACS workstations in a pair-wise comparison of MPR images in three planes for the same patient randomly assigned to right or left monitor.
These were graded on a 5-point Likert-type scale;
−2 Images on left monitor are better than images on right monitor
−1 Images on left monitor are probably better than images on right monitor
0 Images on left and right monitor are equivalent.
+1 Images on right monitor are probably better than left monitor
+2 Images on right monitor are better than left monitor
Quantitative assessment
Two phantoms,
the Catphan®(The Phantom Laboratory,
Salem,
NY,
USA) quality assurance phantom,
with PMMA extension ring,
and the QRM anthropomorphic abdomen phantom (Quality Assurance in Radiology & Medicine,
QRM GmbH,
Moerhrendorf,
Germany),
with extension ring to match patient size,
were scanned on the same scanner with the same acquisition parameters as the patient material (Table 1).
The phantom images were used to measure contrast,
standard deviation (SD),
contrast-to-noise ratio (CNR),
spatial resolution (line pairs/cm) and noise-power spectra (NPS).
Statistical analysis
The scores from reader ratings where analyzed using Visual Grading Regression (VGR) [6] with the regression coefficients in a mixed-effects ordinal logistic model describing the effect of tube load,
IR strength and slice thickness.
The potential dose reduction was estimated by relating two of the regression coefficients to each other [7].