dose efficiency [3]. Image quality is adjusted by noise index (i.e., 1 SD of CT number) with this technique. Use of a high noise index de- creases the radiation ...
M e d i c a l P hy s i c s a n d I n f o r m a t i c s • O r i g i n a l R e s e a r c h Watanabe et al. Image Quality of Low-Dose Abdominal CT Medical Physics and Informatics Original Research
Improvement of Image Quality of Low Radiation Dose Abdominal CT by Increasing Contrast Enhancement Haruo Watanabe1 Masayuki Kanematsu1,2 Toshiharu Miyoshi2 Satoshi Goshima1 Hiroshi Kondo1 Noriyuki Moriyama3 Kyongtae T. Bae 4 Watanabe H, Kanematsu M, Miyoshi T, et al.
Keywords: abdominal CT, contrast enhancement, image noise, image quality, radiation dose DOI:10.2214/AJR.10.4456 Received February 16, 2010; accepted without revision March 10, 2010. 1 Department of Radiology, Gifu University Hospital, 1-1 Yanagido, Gifu 501-1194, Japan. Address correspondence to H. Watanabe. 2
Radiology Services, Gifu University Hospital, Gifu, Japan.
3
Research Center for Cancer Prevention and Screening, National Cancer Center Hospital, Tsukiji, Japan.
4 Department of Radiology, University of Pittsburgh Medical Center, Pittsburgh, PA.
AJR 2010; 195:986–992 0361–803X/10/1954–986 © American Roentgen Ray Society
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OBJECTIVE. The purpose of this study was to evaluate the effects of noise index and contrast material dose on radiation dose, contrast enhancement, image noise, and image quality in abdominal CT. SUBJECTS AND METHODS. Contrast-enhanced abdominal CT with tube current modulation was performed on 195 patients. The patients were prospectively randomized into three groups of equal size (protocol A, noise index of 12 HU and 521 mg I/kg; protocol B, 15 HU and 521 mg I/kg; protocol C, 15 HU and 600 mg I/kg). Scanning was initiated 5 and 45 seconds after aortic enhancement reached 100 HU. Attenuation was measured in the aorta, portal vein, and liver. Transverse CT images were qualitatively graded for diagnostic acceptability and image noise. Arterial phase volume-rendered and multiplanar reformatted (MPR) images and portal venous phase MPR CT angiograms were qualitatively graded for depiction of vessels. Contrast enhancement, objective image noise, radiation dose, and qualitative grades were analyzed and compared among the three groups. RESULTS. The contrast enhancement values of the aorta, portal vein, and liver were higher in protocol C than in protocols A and B (p < 0.05). Objective image noise was greater in protocols B and C than in protocol A (p < 0.05). The radiation dose in protocols B and C was 31–32% lower than in protocol A (p < 0.001). Depiction of vessels, diagnostic acceptability, and subjective image noise were comparable in protocols A and C. CONCLUSION. Use of higher contrast enhancement can compensate for the degradation of image quality resulting from use of a low radiation dose for CT.
T
he amount of CT-derived radiation received by patients has become a growing concern. Especially in the care of patients with chronic diseases, who frequently undergo repeated CT studies during the course of diagnostic imaging and treatment follow-up, it is important to keep CT radiation doses at the minimum needed to obtain diagnostically adequate image quality in clinical practice. Use of low tube voltage for CT reduces radiation dose without degrading image quality and increases the contrast enhancement of vascular and parenchymal structures [1, 2]. However, the increased image noise that accompanies low tube voltage can hamper lesion detectability in the abdomen and pelvis, where tissue contrast is intrinsically low [3]. With automatic tube current modulation, the tube current is automatically adjusted to individual anatomic variations and quantum noise in projections to maintain a selected level of noise on the image and improve radiation
dose efficiency [3]. Image quality is adjusted by noise index (i.e., 1 SD of CT number) with this technique. Use of a high noise index decreases the radiation dose but usually degrades image quality. Although this trade-off is well known, no study, to our knowledge, has been performed to determine how this trade-off is affected by the amount of contrast material. We postulated that administration of a larger amount of contrast medium can compensate for degradations of abdominal CT image quality at higher noise indexes. Thus the purpose of our study was to prospectively evaluate the effect of varying noise index and iodine mass on radiation dose, contrast enhancement, image noise, and image quality in contrast-enhanced abdominal CT. Subjects and Methods Patients This prospective study was approved by our institutional review board, and written informed consent was obtained from all patients. During
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Image Quality of Low-Dose Abdominal CT the 4-month period July through October 2009, 226 consecutively registered patients with clinically or radiologically suspected abdominal disease underwent contrast-enhanced CT of the abdomen. CT was indicated because additional information was needed for clinical diagnostic workup or for evaluation of vascular structures. Thirty-one patients were excluded: nine had undergone total gastrectomy, eight had fatty liver of moderate degree or worse (liver attenuation < 40 HU), seven had undergone partial hepatectomy, three had numerous liver metastatic lesions, two had undergone total splenectomy, and two experienced technical failure related to contrast injection. These patients were excluded because of concerns that splanchnic vascular alterations, abnormal hepatic density, or artifacts might adversely affect evaluation of contrast enhancement and vessel evaluations in our study. The other 195 patients (111 men, 84 women; age range, 29–87 years; mean, 62.6 years; body weight range, 29–88 kg; mean, 58.0 kg) composed the study cohort.
onds after the start of contrast injection. Breathhold scanning for the arterial and portal venous phases was begun 5 and 45 seconds after the bolus-tracking program detected an increase in attenuation reaching 100 HU.
Image Reconstruction CT angiographic (CTA) images were postprocessed at an independent workstation (Advantage Windows 4.4, GE Healthcare). Volume-rendered, arterial phase CTA, and arterial and portal venous phase multiplanar reformations (MPR) were generated. We used a standard volume-rendering reconstruction technique for all patients. The z-axis was selected for cinematic 3D observation at every 10° over 360° of rotation, resulting in 36 volume-rendered images. For all patients MPRs were reconstructed in the coronal plane with 2.5-mm section thickness and no intersection gap by use of the maximum intensity projection method, which resulted in 60–80 MPRs.
Scanning Protocols and Contrast Injection
Quantitative Image Analysis
A 16-MDCT scanner (LightSpeed 16, GE Healthcare) with a fixed tube voltage of 120 kVp and an automatic tube current modulation program (3D mA Modulation, GE Healthcare) was used. This program combined both z-axis and angular modulation of the x-ray tube current (in milliamperes) adjusted for the size and shape of individual patients monitored with a single scout scan, accounting for all three dimensions [4]. Patients were randomized to undergo CT performed with one of the following three protocols. Protocol A had a noise index of 12 HU and a contrast dose of 521 mg I/kg body weight. Protocol B had a noise index of 15 HU and a contrast dose of 521 mg I/ kg. Protocol C had a noise index of 15 HU and a contrast dose of 600 mg I/kg. The minimal and maximal tube current thresholds were 50 and 400 mA. The other CT parameters (collimation, 1.25 mm; detector configuration, 16 × 1.25 mm; table feed, 27.5 mm/rotation; pitch, 1.37; craniocaudal scan range, 25-cm; field of view, 32 × 32 cm; gantry rotation time, 0.5 second; acquisition time, 7.5 seconds) were kept the same for the three protocols. All transverse CT images were reconstructed at 5-mm section thickness with a standard reconstruction algorithm. Nonionic iodinated contrast material containing 300 mg I/mL (iohexol) was administered with a power injector at a fixed injection duration of 30 seconds through a 21-gauge plastic IV catheter typically placed in an antecubital vein. A bolus-tracking program (SmartPrep, GE Healthcare) with a circular region of interest in the upper abdominal aorta was used to begin diagnostic CT scanning. Real-time low-dose (120 kVp, 10 mA) serial monitoring scanning was initiated 5 sec-
A radiologist with 5 years of posttraining experience in interpreting body CT images measured the mean CT attenuation and 1 SD in the abdominal aorta, portal vein, and liver on a DICOM viewer. Measurements were performed on unenhanced, arterial phase, and portal venous phase axial images. Focal hepatic lesions, calcifications, and artifacts were carefully excluded from the measurement areas in evaluation of the liver, blood vessels, and bile ducts. Quantitative degrees of contrast enhancement were expressed as change in attenuation from unenhanced to contrast-enhanced axial images. For each patient, maximal abdominal anteroposterior and transverse dimensions were measured on unenhanced axial images at the level of the upper pole of the right kidney.
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Qualitative Image Analysis Two radiologists (13 and 10 years of posttraining experience in interpreting body CT images) who were unaware of patient clinical information
and CT parameters prospectively and independently reviewed the CT images. Each radiologist graded the images alone first, and then a consensus grade was reached after discussion. The radiologists independently graded the unenhanced, arterial phase, and portal venous phase CT images separately for diagnostic acceptability and subjective image noise on a 5-point scale (1, unacceptable; 2, suboptimal; 3, acceptable; 4, good; 5, excellent). A qualitative grade was comprehensively given after review of all images. Regarding diagnostic acceptability, grade 5 was assigned when the soft-tissue contrast, sharpness of tissue interfaces, lesion conspicuity, and image degradation (caused by streaking noise or beam-hardening artifacts) were deemed superb; grade 3 when image quality was fair and did not hamper image interpretation; and grade 1 when they were considerably deteriorated and hampered image interpretation. Grade 4 corresponded to the intermediate quality between grades 5 and 3, and grade 2 corresponded to that between 1 and 3. For subjective image noise, grade 5 was assigned when minimal or no appreciable mottle or graininess was present, and perception of small anatomic structures, such as blood vessels, lymph nodes, and interfaces between structures was superb. Grade 3 was assigned when some mottling or graininess was appreciable but did not hamper image interpretation. Grade 1 was scored when substantial mottle or graininess hampered image interpretation. Grade 4 corresponded to the intermediate quality between grades 5 and 3, and grade 2 reflected a quality between grades 1 and 3. When reviewing arterial phase volume-rendered and MPR CTA images, the radiologists evaluated the degree of depiction of the common hepatic, proper hepatic, splenic, left gastric, gastroduodenal, dorsal pancreatic, inferior phrenic, superior mesenteric, inferior mesenteric, adrenal, renal, intercostal, and lumbar arteries. When reviewing portal venous phase MPR CTA images, the radiologists assessed the degrees of depiction of the portal, splenic, hepatic, superior mesenter-
TABLE 1: Patient Age, Body Weight, Body Size, and Injection Rate in Three Protocols Characteristic
Protocol A
Protocol B
Protocol C
Age (y)
62.6 ± 9.9
63.6 ± 10.7
61.6 ± 11.1
Body weight (kg)
57.4 ± 11.1
57.0 ± 11.0
59.6 ± 10.2
Anteroposterior dimension (mm)
190.2 ± 29.0
192.2 ± 31.9
194.5 ± 28.6
Transverse dimension (mm)
289.5 ± 31.8
287.6 ± 36.1
294.7 ± 29.1
Injection rate (mL/s)
3.3 ± 0.6
3.3 ± 0.6
3.9 ± 0.6a
Note—Data are mean ± 1 SD. Protocol A, 12-HU noise index and 521 mg I/kg; protocol B, 15-HU noise index and 521 mg I/kg; protocol C; 15-HU noise index and 600 mg I/kg. No significant difference was found in age, body weight, or anteroposterior or transverse dimensions among the three protocols. aValue significantly higher than with protocols A and B (p < 0.001).
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Watanabe et al.
* *
Protocol A Protocol B
250
Protocol A
Protocol C
Protocol B
200
* *
150
* *
100
Protocol C
* *
50 0 Aorta Arterial Phase
Aorta
Objective Image Noise (HU)
Contrast Enhancement (∆HU)
300
20
ic, gastrocolic, renal, and adrenal veins. Arterial and venous trunks and their branches were evaluated separately. Degrees of depiction were graded on a 5-point scale (1, no depiction; 2, faint; 3, acceptable; 4, good; 5, excellent).
CT Dose Index Measurements Volume CT dose index (CTDIvol) in milligrays was documented from the console of the CT scanner. These values were based on weighted CT dose indexes estimated with polymethylmethacrylate head and body phantoms. CTDIvol is the best estimate of the average dose at a given point of scan volume for a given scanning protocol.
* *
* *
988
* *
* *
* *
* *†
* *
10 5 0
Portal Liver Vein Unenhanced
Aorta
Aorta
Portal Liver Vein Arterial Phase
Aorta
Portal Liver Vein Portal Venous Phase
Fig. 2—Chart shows mean objective image noise of abdominal aorta, portal vein, and liver in three protocols. Asterisk indicates significant difference (p < 0.001) between mean objective image noise values; dagger, significant difference (p = 0.033) between mean objective image noise values.
considered to indicate slight agreement; 0.21–0.40, fair agreement; 0.41–0.60, moderate agreement; 0.61–0.80, substantial agreement; and 0.81 or greater, almost perfect agreement [5].
Results Patient Background Factors Each of the three groups who underwent protocols A, B, and C consisted of 65 patients. No significant difference was found between groups in terms of age, body weight, or anteroposterior or transverse dimensions (Table 1). The mean injection rate was higher in protocol C (3.9 ± 0.6 mL/s) than in protocols A and B (both 3.3 ± 0.6 mL/s) (p < 0.001).
Statistical Analysis Quantitative Image Analysis Contrast enhancement expressed as change in attenuation of the abdominal aorta, portal
vein, and liver in the three protocols is summarized in Figure 1. In the arterial phase, mean change in attenuation of the aorta was greater with protocol C than with protocols A and B (p < 0.001). In the portal venous phase, the mean change in attenuation of the aorta, portal vein, and liver was greater with protocol C than with protocols A and B (p < 0.05). Objective image noise expressed as 1 SD of attenuation measured over each region of interest is summarized in Figure 2. The means of objective image noise of the aorta, portal vein, and liver during the unenhanced, arterial, and portal venous phases were greater with protocols B and C than with protocol A (p < 0.001), and the mean objective image noise of the portal vein during the portal venous phase was greater with protocol C than with protocol B (p = 0.033). Protocol A Protocol B Protocol C
Fig. 3—Chart shows mean diagnostic acceptability and subjective image noise grades for three protocols. Degradation of image quality with protocol B (noise index, 15 HU; 521 mg I/kg) was compensated for by increasing iodine load in protocol C (noise index, 15 HU; 600 mg I/kg). Asterisk indicates significant difference (p < 0.01) between mean grades.
5 4 Grade
Statistical analyses were performed with software (SPSS version 17, SPSS). For quantitative measurements, one-way analysis of variance was performed to compare background factors (patient age, body weight, anterior and transverse abdominal dimensions, and injection rate), change in attenuation, objective image noise, and CTDIvol. When a statistically significant difference was found in the three protocols, pairwise comparisons were performed with the Tukey test, and p < 0.05 was considered significant. The Kruskal-Wallis test was used to compare the qualitative grades recorded by the two radiologists. When a significant difference was found in the three protocols, pairwise comparisons were performed with the Mann-Whitney test, and a stricter p < 0.017 was considered significant, introducing the Bonferroni correction. To assess interobserver variability, kappa statistics were used to quantify the degree of agreement. A kappa value up to 0.20 was
* *
15
Portal Vein Liver Portal Venous Phase
Fig. 1—Chart shows mean contrast enhancement of abdominal aorta, portal vein, and liver in three protocols. Contrast enhancement of abdominal aorta, portal vein, and liver with protocol C was greater than with protocols A and B (p < 0.05).
* *
* *
**
*
* *
*
**
3 2 1 0
Portal Venous Phase Diagnostic Acceptability
Unenhanced
Arterial Phase
Unenhanced
Arterial Phase
Portal Venous Phase Subjective Image Noise
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Image Quality of Low-Dose Abdominal CT
A
B
C
Fig. 4—Comparison of subjective image quality and noise during portal venous phase. Subjective image quality and noise are apparently equivalent for three protocols. A, 66-year-old 66-kg man with rectal cancer. Axial CT image obtained with protocol A (volume CT dose index, 12.8 mGy). B, 64-year-old 65-kg man with rectal cancer. Axial CT image obtained with protocol B (volume CT dose index, 8.8 mGy). C, 55-year-old 65-kg man with rectal cancer. Axial CT image obtained with protocol C (volume CT dose index, 8.7 mGy).
Qualitative Image Analysis Mean diagnostic acceptability and subjective image noise grades for the three protocols are shown in Figure 3. For diagnostic acceptability, the mean grade of protocol B during the portal venous phase was lower than that of protocol C (p = 0.002). For subjective image noise, the mean grade of protocol B during the portal venous phase was lower than that of protocols A (p = 0.006) and C (p = 0.001). No significant difference was found between protocols A and C in terms of diagnostic acceptability and subjective image noise during the arterial and portal venous phases (Fig. 4). The degree of depiction of arteries on arterial phase volume-rendered CTA images is summarized in Table 2. No difference between protocols was found in terms of depiction of all arterial trunks and most arterial branches. However, depiction of splenic (p = 0.013) and left gastric (p < 0.001) arterial branches with protocol B was inferior to that with protocol A; depictions of inferior phrenic artery (p < 0.01) and inferior mesenteric arterial branches (p = 0.01) with protocol B were inferior to those with protocols A and C (Fig. 5); depiction of renal arterial branches with protocol C was superior to that with protocols A and B (p < 0.005); and depiction of lumbar arteries with protocol A was superior to that with protocols B and C (p < 0.001). Degrees of depiction of arteries on arterial phase MPR CTA images are summarized in Table 3. No difference between protocols was found for the depictions of all arterial trunks and most arterial branches. However, depiction of splenic (p < 0.001) and superior mesenteric (p < 0.01) arterial branches with protocol B was inferior to that with protocol C; depiction of left gastric arterial branches (p < 0.005), inferior phrenic (p < 0.01), intercostal (p < 0.005), and lumbar (p < 0.001) arteries with protocol B
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TABLE 2: Degree of Depiction of Arteries on Arterial Phase Volume-Rendered CT Angiograms Protocol A
Protocol B
Protocol C
p
Common hepatic artery
Anatomic Site
4.8 ± 0.5
4.8 ± 0.6
4.9 ± 0.2
0.60
Proper hepatic artery
4.4 ± 0.7
4.0 ± 1.0
4.0 ± 0.9
0.14
5.0 ± 0.0
4.9 ± 0.5
4.9 ± 0.3
0.41
3.4 ± 1.0
2.7 ± 0.8a
3.1 ± 0.8
0.040
3.7 ± 1.1
3.4 ± 1.0
3.9 ± 0.9
0.12
2.7 ± 1.4
1.7 ± 0.8a
2.0 ± 1.0
0.001
Trunk
4.2 ± 0.9
4.4 ± 0.8
4.7 ± 0.6
0.061
Branches
Splenic artery Trunk Branches Left gastric artery Trunk Branches Gastroduodenal artery 2.9 ± 0.9
2.6 ± 0.7
2.6 ± 0.6
0.21
Dorsal pancreatic artery
1.6 ± 0.7
1.4 ± 0.8
1.6 ± 0.7
0.10
Inferior phrenic artery
2.1 ± 1.4
1.3 ± 0.6b
1.7 ± 0.7
0.004
Trunk
5.0 ± 0.0
4.9 ± 0.2
5.0 ± 0.2
0.41
Branches
2.6 ± 0.7
3.0 ± 0.7
3.0 ± 0.7
0.030
Trunk
4.1 ± 0.9
3.8 ± 0.9
4.1 ± 0.7
0.37
Branches
2.7 ± 1.4
1.8 ± 0.8b
2.6 ± 0.9
0.002
1.5 ± 0.8
1.2 ± 0.4
1.3 ± 0.4
0.22
4.9 ± 0.4
4.9 ± 0.2
5.0 ± 0.2
0.42 < 0.001
Superior mesenteric artery
Inferior mesenteric artery
Adrenal arteries Renal artery Trunk
2.4 ± 0.6
2.7 ± 0.7
3.2 ± 0.6c
Intercostal arteries
2.4 ± 1.0
2.1 ± 0.6
2.1 ± 0.4
0.31
Lumbar arteries
3.2 ± 1.1d
2.2 ± 0.4
2.3 ± 0.5
< 0.001
Branches
Note—Data are mean ± 1 SD; p values from Kruskal-Wallis test. aValues were significantly lower than with protocol A. bValues were significantly lower than with protocols A and C. cValue was significantly greater than with protocols A and B. d Value was significantly greater than with protocols B and C.
was inferior to that with protocols A and C (Fig. 6); depiction of adrenal arteries with protocol A was superior to that with protocols B and C (p
0.99 0.028
Inferior mesenteric artery
Adrenal arteries Renal artery Trunk
4.9 ± 0.3
5.0 ± 0.0
4.9 ± 0.3
0.21
Branches
3.2 ± 0.6
3.2 ± 0.5
3.6 ± 0.5d
0.002
Intercostal arteries
3.1 ± 0.7
2.6 ± 0.5b
3.1 ± 0.7
0.004
Lumbar arteries
3.6 ± 0.8
2.7 ± 0.6b
3.3 ± 0.6
< 0.001
Note—Data are mean ± 1 SD; p calculated with Kruskal-Wallis test. aValues were significantly lower than with protocol C. bValues were significantly lower than with protocols A and C. cValue was significantly greater than with protocols B and C. d Value was significantly greater than with protocols A and B.
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The degrees of depiction of veins on portal venous phase MPR images is summarized in Table 4. No significant difference was found between protocols in most veins, except that depiction of portal vein branches with protocol B was inferior to that with protocol C (p = 0.008), splenic vein branches with protocol A was superior to that with protocols B and C (p = 0.003), renal vein trunk with protocol B was inferior to that with protocols A and C (p = 0.006), and right adrenal vein with protocol B was inferior to that with protocol A (p = 0.002). Reader Agreement The kappa values of the independent ratings by the two radiologists for depiction of 19 arterial structures with the three protocols ranged from 0.62 to 0.88. That for depiction of 12 venous structures ranged from 0.67 to 0.91, indicating substantial to almost perfect agreement. The kappa values for diagnostic acceptability and image noise ranged from 0.61 to 0.91, also indicating substantial to almost perfect agreement. CTDI The CTDIvol values for protocol B (mean, 7.3 ± 3.1 mGy; range, 2.9–16.1 mGy) and protocol C (mean, 7.5 ± 3.2 mGy; range 3.8– 18.3 mGy) were significantly lower than that of protocol A (mean, 10.8 ± 3.9 mGy; range, 3.4–18.7 mGy) (p < 0.001). Discussion Close associations and trade-offs among noise index, image quality, and radiation exposure are well known. A reduction in noise
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Image Quality of Low-Dose Abdominal CT
A
B
C
Fig. 6—Arterial phase multiplanar reformatted CT angiograms obtained with three protocols (CTDIvol = volume CT dose index). Arrow indicates left gastric arterial branches. A, 79-year-old 62-kg man with malignant lymphoma who underwent imaging with protocol A (CTDIvol, 12.2 mGy). B, 77-year-old 45-kg woman with breast cancer who underwent imaging with protocol B (CTDIvol, 4.1 mGy). Left gastric arterial branches (arrows) are less apparent than in A and C. C, 72-year-old 57-kg man with rectal cancer who underwent imaging with protocol C (CTDIvol, 4.2 mGy).
TABLE 4: Degree of Depiction of Veins on Portal Venous Phase Multiplanar Reformatted Images Anatomic site
Protocol A
Protocol B
Protocol C
p
4.9 ± 0.3
4.8 ± 0.4
4.9 ± 0.2
0.40
3.9 ± 0.8
3.8 ± 0.8a
4.2 ± 0.6
0.032
Trunk
4.9 ± 0.3
4.9 ± 0.4
4.8 ± 0.5
0.72
Branches
3.8 ± 0.9b
3.2 ± 0.5
3.2 ± 0.7
0.001
Hepatic veins
4.5 ± 0.7
4.6 ± 0.7
4.8 ± 0.4
0.095
Trunk
5.0 ± 0.2
5.0 ± 0.0
5.0 ± 0.0
0.41
Branches
3.6 ± 0.7
3.3 ± 0.6
3.4 ± 0.5
0.086
3.5 ± 1.0
3.1 ± 1.0
3.2 ± 1.1
0.312
Trunk
4.8 ± 0.4
4.5 ± 0.6c
4.9 ± 0.4
< 0.001
Branches
2.9 ± 0.8
2.8 ± 0.6
2.9 ± 0.3
0.57
3.4 ± 1.0
3.1 ± 0.8
3.3 ± 0.6
0.11
2.7 ± 1.5
1.7 ± 0.8d
1.9 ± 0.5
0.004
Portal vein Trunk Branches Splenic vein
Superior mesenteric vein
Gastrocolic trunk Renal vein
Left adrenal vein Right adrenal vein
Note—Data are mean ± 1 SD; p calculated with Kruskal-Wallis test. aValue was significantly lower than with protocol C. bValue was significantly greater than with protocols B and C. cValue was significantly lower than with protocols A and C. d Value was significantly lower than with protocol A.
index by increasing tube current improves image quality but increases radiation dose (a 5% reduction in noise index corresponds to an approximately 10% increase in radiation dose) [3, 6]. Conversely, the use of a high noise index (lower tube current output) to reduce radiation dose results in an increase in image noise and a degradation of image quality. Kalra et al. [6] investigated the effect of noise index on image quality and radiation exposure with 16-MDCT.
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They reported that use of a noise index of 15 HU resulted in significantly greater objective image noise but that subjectively evaluated image noise and diagnostic acceptability were not significantly different from those with noise indexes from 10.5 to 15 HU (while reducing radiation dose). In our study, diagnostic acceptability was significantly lower with protocol B (noise index, 15 HU; 521 mg I/kg) than with protocol C (15
HU; 600 mg I/kg). This finding suggests that an increase in iodine mass in the contrast material may compensate for image degradation due to a lower tube current output (higher noise index). Therefore, use of greater contrast enhancement may be an approach to reduction of radiation dose without compromising diagnostic image quality. To our knowledge, no published study to date has explored increasing contrast enhancement to compensate for increased noise due to a reduction in radiation dose. Quantitative degree of contrast enhancement was significantly greater with protocol C (600 mg I/kg) than with the other protocols in all anatomic structures at all phases of contrast enhancement. This result was expected because maximum hepatic enhancement correlates directly with the amount of iodine in contrast material [7]. Despite this increased contrast enhancement, objective image noise (measured as 1 SD of CT number over regions of interest) was unaffected from the preset noise index. This finding indicates that the administration of increased iodine mass does not compensate for increased quantitative image noise due to low tube current output. Kalra et al. [6] reported finding no significant differences in subjectively evaluated image noise and diagnostic acceptability with noise indexes ranging from 10.5 to 15 HU. We, however, observed significant differences in qualitative assessments, particularly during the portal venous phase, in which the noise index increased from 12 to 15 HU at a fixed iodine mass. Subjective image noise was considerably lower with protocol B (noise index, 15 HU; 521 mg I/kg) than with protocol A (noise index, 12 HU; 521 mg I/kg).
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Watanabe et al. The main limitation of low tube voltage technique is the substantial increase in image noise caused by reduced photon flux and energy. Although image noise itself can be related to the specificities of the CT scanner, individual patient factors such as body weight and anteroposterior and transverse dimensions can affect this noise [2, 8]. This increase in image noise should be carefully considered because it can influence diagnostic interpretations, especially in abdominal studies because low-contrast lesion detectability is markedly affected by image noise [3]. Because of the comparable subjective image noise in protocols A and C, our study results suggest that low-contrast lesion detectability would be improved by either reducing the noise index or increasing contrast enhancement. It is essential that the risks of adverse events associated with increasing the amount of iodine administered are properly discussed. Potential adverse effects include anaphylactic shock, allergy-like reactions, and contrast-induced nephropathy. Anaphylactic and allergy-like reactions are most common among patients with asthma, apprehension, a history of contrast reactions, and cardiovascular or renal disease, irrespective of the contrast volume and route of administration [9]. The incidence of contrast-induced nephropathy is less than 2% in the general population but higher among patients with renal impairment or diabetes [10]. In our literature search, we found no published evidence suggesting an increase from 521 mg I/kg to 600 mg I/kg (equivalent to 4 g of iodine for a 50-kg patient) would increase the frequen-
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cy of anaphylactic or allergy-like reactions or contrast-induced nephropathy. Our study had limitations. First, we did not evaluate tumor detection, tumor conspicuity, or tumor-to-liver contrast-to-noise ratio, because of the low disease prevalence in our study population. Second, we used CTDIvol (an index of exposure dose) as a parameter of radiation exposure because evaluations of effective and absorption doses are not feasible in clinical studies. However, although CTDIvol is an international standard parameter for CT radiation dose and has been used widely in clinical studies, there may be substantial discrepancies between CTDIvol values displayed on the scanner and actual measured radiation doses, particularly for obese patients. Finally, our study included patients weighing 29–88 kg, which is a narrower body weight range than would be expected in most Western cultures. Image degradation with CT at a noise index of 15 HU may be more adversely affected, particularly in imaging of overweight patients in Western populations, even with administration of 600 mg I/kg. Our study showed that in terms of subjective image quality and vessel depiction with CTA, CT scans acquired with higher contrast enhancement at 30% reduction in radiation dose are comparable to CT scans obtained with a full radiation dose. References 1. Marin D, Nelson RC, Samei E, et al. Hypervascular liver tumors: low tube voltage, high tube current multidetector CT during late hepatic arterial
phase for detection—initial clinical experience. Radiology 2009; 251:771–779 2. Nakayama Y, Awai K, Funama Y, et al. Abdominal CT with low tube voltage: preliminary observations about radiation dose, contrast enhancement, image quality, and noise. Radiology 2005; 237:945–951 3. Kalra MK, Maher MM, Toth TL, et al. Strategies for CT radiation dose optimization. Radiology 2004; 230:619–628 4. Kalra MK, Maher MM, Toth TL, et al. Techniques and applications of automatic tube current modulation for CT. Radiology 2004; 233:649–657 5. Landis JR, Koch GG. The measurement of observer agreement for categorical data. Biometrics 1977; 33:159–174 6. Kalra MK, Maher MM, Kamath RS, et al. Sixteen-detector row CT of abdomen and pelvis: study for optimization of Z-axis modulation technique performed in 153 patients. Radiology 2004; 233:241–249 7. Heiken JP, Brink JA, McClennan BL, Sagel SS, Crowe TM, Gaines MV. Dynamic incremental CT: effect of volume and concentration of contrast material and patient weight on hepatic enhancement. Radiology 1995; 195:353–357 8. Huda W. Dose and image quality in CT. Pediatr Radiol 2002;32:709–713; discussion 751–754 9. Singh J, Daftary A. Iodinated contrast media and their adverse reactions. J Nucl Med Technol 2008;36:69–74; quiz 76–67 10. Nguyen SA, Suranyi P, Ravenel JG, et al. Isoosmolality versus low-osmolality iodinated contrast medium at intravenous contrast-enhanced CT: effect on kidney function. Radiology 2008; 248:97–105
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