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Tin-filter Enhanced Dual-Energy-CT

Posted Apr 30, 2013 Updated Apr 10, 2023
By Sascha Kaufmann MD , Alexer Sauter MD , Daniel Spira MD , Sergios Gatidis MD , Dominik Ketelsen MD , Martin Heuschmid MD , Claus D. Claussen MD , Christoph Thomas MD
11 min read

Objectives

To measure and compare the objective image quality of true noncontrast (TNC) images with virtual noncontrast (VNC) images acquired by tin-filter–enhanced, dual-source, dual-energy computed tomography (DECT) of upper abdomen.

Materials and Methods

Sixty-three patients received unenhanced abdominal CT and enhanced abdominal DECT (100/140 kV with tin filter) in portal-venous phase. VNC images were calculated from the DECT datasets using commercially available software. The mean attenuation of relevant tissues and image quality were compared between the TNC and VNC images. Image quality was rated objectively by measuring image noise and the sharpness of object edges using custom-designed software. Measurements were compared using Student two-tailed t -test. Correlation coefficients for tissue attenuation measurements between TNC and VNC were calculated and the relative deviations were illustrated using Bland-Altman plots.

Results

Mean attenuation differences between TNC and VNC (HU TNC − HU VNC ) image sets were as follows: right liver lobe −4.94 Hounsfield units (HU), left liver lobe −3.29 HU, vena cava −2.19 HU, spleen −7.46 HU, pancreas 1.29 HU, fat −11.14 HU, aorta 1.29 HU, bone marrow 36.83 HU (all P < .05); right kidney 0.46 HU, left kidney 0.56 HU, vena portae −0.48 HU and muscle −0.62 HU (nonsignificant). Good correlations between VNC and TNC series were observed for liver, vena portae, kidneys, pancreas, muscle and bone marrow (Pearson’s correlation coefficient ≥0.75). Mean image noise was significantly higher in TNC images ( P < .0001). Measurements of edge sharpness revealed no significant differences between VNC and TNC images ( P = .19).

Conclusion

The Hounsfield units in VNC images closely resemble TNC images in the majority of the organs of the upper abdomen (kidneys, liver, pancreas). In spleen and fat, Hounsfield numbers in VNC images are tend to be higher than in TNC images. VNC images show a low image noise and satisfactory edge sharpness. Other criteria of image quality and the depiction of certain lesions need to be evaluated additionally.

The idea of dual-energy computed tomography (DECT) has existed since the invention of CT . However, only recently CT technology has advanced far enough to enable dose-neutral DECT acquisitions . Material differentiation in DECT is based on the dependence of photo absorption on energy and atomic number. If CT data that were acquired with different energies are available, a differentiation of the effective atomic numbers of the scanned materials and thus a differentiation of materials with different atomic numbers is feasible . Besides other possible technical approaches, dual-source CT with its two x-ray tubes can supply two simultaneous CT acquisitions with different energy levels . The energetic difference between the two spectra can be enhanced using a tin filter . The filter leads to a sharpening of the high-energy spectrum and thus a higher mean energy level of this spectrum and more effective dual energy discrimination. Thus, it is possible to use of 100 kV instead of 80 kV to create the lower energy spectrum that leads to an improved image quality of the low-energy images especially in the abdomen . Several clinical applications of DECT have recently been investigated . Because of the high atomic number of iodine, the amount of iodine contributing to every voxel can be quantified in DECT datasets using a three-material decomposition algorithm . Thus, the iodine content can be subtracted from the contrast enhanced images, resulting in virtual noncontrast (VNC) images.

Nonenhanced scans can be of interest for the measurement of contrast uptake of lesions, for imaging metabolic changes of liver tissue, and for the detection of calcifications, hematoma, and hemorrhagic cysts. However, because the unenhanced scan is not necessary in many patients, many radiology centers tend to abstain from acquiring the additional images for reasons of dose reduction. Unfortunately, once contrast is administered, the acquisition of noncontrast image is no longer possible in a reasonable time interval, which can hamper the interpretation of incidental findings.

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Materials and methods

Patient Population

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CT Scanning Protocol

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Image Reconstruction and Postprocessing

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Measurement of Hounsfield Numbers for TNC and VNC

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Table 1

Summary of Measurement Results

Organ HU TNC HU VNC HU TNC −HU VNC P Value Correlation Coefficient Right liver lobe52.84 ± 9.1857.78 ± 10.10−4.94 ± 3.86<.00010.92 ( P < .0001) Left liver lobe53.68 ± 9.0256.97 ± 9.55−3.29 ± 3.79<.00010.92 ( P < .0001) Vena portae 38.56 ± 5.18 39.03 ± 6.29 −0.48 ± 3.32 .2587 0.85 ( P < .0001) Vena cava38.03 ± 5.3835.84 ± 5.382.19 ± 4.42.00120.66 ( P < .0001) Right kidney 29.97 ± 3.32 29.51 ± 4.63 0.46 ± 2.88 .2091 0.79 ( P < .0001) Left kidney 30.03 ± 3.28 29.48 ± 4.40 0.56 ± 2.88 .1313 0.75 ( P < .0001) Spleen45.57 ± 3.5553.16 ± 5.58−7.46 ± 5.38<.00010.38 ( P = .0026) Pancreas37.76 ± 6.7936.48 ± 7.141.29 ± 3.67.00720.86 ( P < .0001) Fat−100.10 ± 27.96−88.95 ± 27.81−11.14 ± 9.26<.00010.94 ( P < .0001) ∗ Muscle 45.48 ± 10.03 46.10 ± 10.49 −0.62 ± 3.88 .21 0.93 ( P < .0001) Aorta39.02 ± 5.1637.17 ± 5.391.29 ± 3.67.00270.61 ( P < .0001) Bone marrow134.62 ± 61.5279.95 ± 46.5636.83 ± 46.31<.00010.96 ( P < .0001)

HU, Hounsfield unit; TNC, true noncontrast; VNC, virtual noncontrast.

P values refer to paired sample t -tests. Statistically significant differences are printed in bold face.

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Objective Measurements of Image Quality

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Figure 1, For measurement of edge sharpness the border between hepatic surface and visceral fat was evaluated by manually drawing a line perpendicular to the liver surface ( left side ). The software ( right side ) measures the slope of the steepest increase of Hounsfield numbers in this line ( black line ). ( Red line ) Original Hounsfield numbers; ( green line ) filtered attenuation values. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Radiation Dose Evaluation

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Statistical Analysis

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Figure 2, (a–l) The Bland-Altman plots depicting the differences in Hounsfield unit (HU) measurements between true noncontrast (TNC) and virtual noncontrast (VNC) revealed homogenous distribution of the HU deviation for all analyzed tissues except for bone marrow, in which the deviation increased with increasing HU.

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Results

Hounsfield Numbers

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Table 2

Comparison of the Differences in the CT Numbers of TNC and VNC Images (TNC-VNC) in Recently Published Studies

Organ Graser et al Altenbernd et al Zhang et al Toepker et al Barrett et al Own Data Average Year 2009 2010 2010 2011 2011 2012 DS scanner generation First First First Second Second Second Tube potentials (kVp) 140/80 140/80 140/80 140/100 140/100 140/100 Quality reference mAs 96/404 96/404 96/404 178/230 178/230 151/196 Mean CTDI vol Not given Not given 8.2 12.9 11.0 11.2 Number of patients 110 40 102 86 75 63 476 Right liver lobe −2.0 ( P = .09) −3.3 ( P = .000) 0.3 ( P = .856) −5.7 ( P < .001) −2.7 ( P < .001) −4.94 ( P < .0001) −2.7 Left liver lobe −3.29 ( P < .0001) Right kidney 0.8 ( P = .26) — — −0.1 ( P = .883) — 0.46 ( P = .2091) 0,4 Left kidney — — — 0.56 ( P = .1313) Spleen — — −0.5 ( P = .062) −6.3 ( P < .001) — −7.46 ( P < .0001) −4.3 Pancreas — — 2.9 ( P = .089) −2.4 ( P < .001) — 1.29 ( P = .0072) −1.0 Fat — −11.3 ( P = .000) −14.1 ( P < .001) −7 ( P < .001) — −11.14 ( P < .0001) −11.0 Muscle −1.6 ( P = .42) −1.4 ( P = .519) 0.5 ( P = .544) −2.8 ( P < .001) — −0.62 ( P = .21) −1.1 Aorta 0.9 ( P = .16) −0.3 ( P = .88) 6.0 ( P < .001) −2.6 ( P < .001) — 1.84 ( P = .0027) −0.6

CT, computed tomography; CTDI vol , volume CT dose index; DS, dual source; TNC, true noncontrast; VNC, virtual noncontrast.

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Objective Image Quality

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Figure 3, A 50-year-old female patient with a history of breast cancer. TNC images (a) , VNC images (b) , and contrast-enhanced images (c) . The VNC image clearly shows lower image noise than the TNC image. The hepatic lesion ( arrow ) is clearly depicted in the VNC image. TNC, true noncontrast; VNC, virtual noncontrast.

Figure 4, A 44-year-old male patient with acute abdomen. TNC images (a) , VNC images (b) , and contrast-enhanced images (c) all show the concrement in the right kidney ( arrow ). The stone appears smaller and less dense in the VNC image. TNC, true noncontrast; VNC, virtual noncontrast.

Figure 5, A 66-year-old male patient with acute abdomen. TNC images (a) , VNC images (b) , and contrast-enhanced images (c) all show a calcified lesion of the pancreas corpus and a cystic lesion in the pancreas tail ( arrow ). In the VNC images, the lesion and the lesion wall can be discriminated easier than in the TNC image. Intraluminal and intravasal contrast medium has been removed. TNC, true noncontrast; VNC, virtual noncontrast.

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Radiation Dose

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Discussion

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Limitations

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Conclusions

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Acknowledgments

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Academic Radiology, Volume 20, Issue 5
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