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Low kVp scanning of pulmonary embolism patients using Iterative Model Reconstruction

Philips CT Clinical Science Philips Healthcare • USA

Purpose of study
The purpose of the study was to evaluate dose reduction and image quality of CT pulmonary angiography (CTPA) scans, using 80 kVp and reconstructed with IMR, as compared with scans of 100 kVp using iDose⁴.

The following is a summary of the study published in Emergency Radiology by Dr. Ippolito and colleagues from San Gerardo Hospital in Italy.

Acute pulmonary embolism (PE) is a common, life-threatening condition that can result in significant morbidity and mortality in adults. CT pulmonary angiography (CTPA) is the most commonly used imaging procedure for patients suspected of having PE. With the increasing use of CT angiography, there have been growing concerns over patient exposure to ionizing radiation, leading to the application of various dose reduction strategies. One of the methods that has been considered for dose reduction is low kilovoltage (kV) or milliampere second (mAs) settings. However, such reductions in radiation dose are hindered by increased image noise and degraded image quality from the standard filtered back-projection (FBP) reconstruction algorithm currently used on most CT systems. However, the introduction of iterative reconstruction methods such as IMR have resulted in a significant noise reduction in CT. IMR may also improve other aspects of image quality such as spatial resolution, low contrast detectability and edge sharpness. The purpose of the study was to determine whether a model-based iterative reconstruction such as IMR can maintain high diagnostic image quality with reduction of the radiation dose in patients undergoing CTPA.

The study included 151 patients divided into two groups, with 76 patients evaluated using 80 kVp with IMR, and 75 patients evaluated using the 100 kVp setting with iDose⁴. All patients were scanned with the same scanning and contrast injection protocol. Images were analyzed in both a quantitative and qualitative manner. Quantitative evaluation was performed manually by placing a circular region of interest in the center of a vessel at three different locations in the main pulmonary artery, right pulmonary artery and left pulmonary artery. Size of ROI was adjusted depending on the vessel size. Background noise was measured by drawing ROIs in the air surrounding the patient. Muscle attenuation was measured by drawing ROIs in paraspinal and subscapular muscles. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were measured using background and muscle ROIs. Qualitative evaluation was performed by two experienced readers using a 4-point scale, with 4 regarded as excellent, 3 as good, 2 as acceptable, and 1 as low.

The effective radiation dose (ED) in the IMR group was 3.48 ± 1.2 mSv, and in the iDose⁴ group, it was 4.93 ± 1.8 mSv, resulting in a savings of 29% in the IMR group. CNR was significantly higher for the IMR group as compared to the iDose⁴ group, with average CNR nearly 6.5 times higher using IMR (CNR = 58.9 ± 18.13) than iDose⁴ (CNR = 9.19 ± 4.34). Similar results were observed with SNR (mean value of 61.6 ± 15.1 vs. 18.73 ± 9.84). Also, the combination of 80 kVp and IMR allowed for improvement in the iodine signal from the pulmonary arteries. The subjective image quality of all patients was considered by both readers as diagnostic, with a mean value of 3.28 ± 0.79 in the IMR group as compared to 2.62 ± 0.72 in the iDose⁴ group. Improved visualization of small peripheral pulmonary arteries was achieved, and a significant improvement was seen for the delineation of the sub-segmental arteries.

Low kVp scanning with IMR allows a significant radiation dose reduction in CTPA patients while improving image quality. SNR and CNR values in patients reconstructed with IMR were significantly higher as compared to iDose4. IMR algorithm allows a reduction in the tube voltage and tube current in comparison with iDose⁴, enabling a significant decrease in radiation exposure in terms of CTDIvol (33%), DLP (30%), and ED (29% mSv), while providing CTPA images with a higher diagnostic quality.

Clinical relevance
IMR allows for diagnostically acceptable low-dose CT images without compromising image quality in CTPA patients in emergency departments.

Ippolito, Davide, et al. “Evaluation of Image Quality and Radiation Dose Saving Comparing Knowledge Model–Based Iterative Reconstruction on 80-KV CT Pulmonary Angiography (CTPA) with Hybrid Iterative Reconstruction on 100-KV CT.” Emergency Radiology, 10 Nov. 2018, doi:10.1007/s10140-018-1653-4.

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Apr 8, 2019

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Body, chest, CTPA, FBP reconstruction, iDose4, image quality, IMR, iterative model reconstruction, PE, pulmonary arteries, Vascular

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