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. 2006 Jan;27(1):40-5.

Effect of CT acquisition parameters in the detection of subtle hypoattenuation in acute cerebral infarction: a phantom study

C Tanaka  1 , T Ueguchi, E Shimosegawa, N Sasaki, T Johkoh, H Nakamura, J Hatazawa
Affiliations

Effect of CT acquisition parameters in the detection of subtle hypoattenuation in acute cerebral infarction: a phantom study

C Tanaka et al. AJNR Am J Neuroradiol. 2006 Jan.

Abstract

Background and purpose: We evaluated the effects of varying tube voltage, current per rotation, and section thickness on detectability of 2- and 4-Hounsfield unit (HU) differences on brain CT between normal and ischemic gray matter within 6 hours of ischemia onset, by using a low-contrast phantom.

Methods: The phantom with an attenuation of 36 HU corresponding to normal gray matter contained 2 sets of spheres (34 HU and 32 HU) corresponding to the early CT signs of ischemic brain and complete infarction, respectively. The reproducibility of the CT numbers and the contrast-to-noise ratio (CNR), defined as the CT number difference between the background (36 HU) and the spheres (34 HU or 32 HU) divided by the SD of the background CT number were measured. Five radiologists rated the phantom images for detection of the low-contrast spheres by visual inspection.

Results: The CT numbers were reproducible within 1 HU with a tube current of > or =150 mAs at 120 kVp. The CNRs for the 34- and 32-HU spheres were positively correlated with the tube voltage, tube current per rotation, and the section thickness. A CNR of 1.0 was obtained for the 34-HU sphere when scanning was conducted with a section thickness of 10 mm at 120 kVp and 700 mAs, or 135kVp and 450 mAs, respectively. A significant improvement of the accuracy of detection was found with increasing tube current, tube voltage per rotation, and section thickness.

Conclusion: Our study indicated that the 2-HU hypoattenuation corresponding to the early CT sign of acute ischemic stroke can be detected by using appropriate parameter settings.

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Figures

Fig 1.
Fig 1.
Low-contrast phantom. Phantom overview before CT scanning (A) and representative phantom image (window width, 100 HU; window level, 35 HU) (B). Two sets of 3-, 2-, and 1-cm-diameter acrylic resin spheres were embedded in an 18-cm-diameter cylindrical phantom. The attenuation was adjusted to 32 HU in one set of spheres (arrowheads), corresponding to complete cerebral infarction, and 34 HU in the other set of spheres (arrows), corresponding to early ischemic brain damage. The attenuation of the cylindrical phantom was 36 HU, corresponding to normal gray matter.
Fig 2.
Fig 2.
Reproducibility of the HU values. The HU values obtained in 14 examinations performed on different days were plotted against various tube currents. The tube voltages were (A) 135 kVp, (B) 120 kVp, (C) 100 kVp, and (D) 80 kVp. The section thickness was fixed at 5 mm for all the measurements.
Fig 3.
Fig 3.
Effect of tube voltage, tube current, and section thickness on the contrast-to-noise ratio (CNR). The section thicknesses were (A) 5 mm, (B) 10 mm, and (C) 20 mm, respectively. The CNRs obtained at different tube voltages were plotted against the tube current.
Fig 3.
Fig 3.
Effect of tube voltage, tube current, and section thickness on the contrast-to-noise ratio (CNR). The section thicknesses were (A) 5 mm, (B) 10 mm, and (C) 20 mm, respectively. The CNRs obtained at different tube voltages were plotted against the tube current.
Fig 4.
Fig 4.
Effect of tube current on the CNR in the 34-HU and 32-HU spheres. The CNRs for the 32-HU and the 34-HU spheres were separately plotted against the tube current. The tube voltage and section thickness were fixed at 120 kVp and 5 mm, respectively.
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