Current Clinical Concerns in CT: Results : Protocols

HRCT Lung

1. A 16 slice MSCT (Philips MX 8000 IDT) has recently been installed in our department and I would like to ask the following questions. First, how can I manage cases of diffuse lung disease (I used to do HRCT with serial technique 1 mm/10 or 20 mm and four expiration scans in preselected regions with 1 mm thickness) in order to keep the quality of HRCT and the radiation low?

2. How often do you ask for HRCT with inspiration AND expiration, in order to decide between ground glass and air-trapping (approximately in %)

3. What is your opinion on the issue of sequential scanning for high resolution CT vs. whole chest volumetric acquisition and thin section high resolution reconstruction? The does is obviously lower with the nonspiral scans at 10 mm intervals, but I notice less problems with motion artifact when doing a single breathhold whole chest scan. And obviously a whole chest scan gives you a whole chest scan. If the indication is evaluate for ILD, is it ethical to radiate the whole chest? We are beginning to do more whole lung screening with 16 MDCT and just lower the mAs. We then reconstruct data with both High resolution and standard technique.

4. Until recently our hi-res chest protocol for children with ? bronchiectasis was for inspiratory run only. A visiting radiologist said we should be doing a few expiratory slices as well. In light of the fact that expiratory images would incur a higher dose, we are interested to know what the current thinking is on this for pediatrics.

5. I work at a remote site and my radiologists are located in another city. I frequently get requests for HRCT with contrast. Is this an appropriate request and in what clinical circumstances is it valuable? Shouldn't HRCT of the chest be done without contrast? You thought spiral vs. sequential?

References
Kelly DM, Hasegawa I, Borders R, Hatabu H and Boiselle PM. High-resolution CT using MDCT: Comparison of degree of motion artifacts between volumetric and axial methods. AJR 2004; 182: 757-759.

• Summary: This study compared volumetric datasets and axial nonvolumetric datasets from 4 (N=40) and 8 (N=7) slice MDCT scanners. Volumetric datasets were reconstructed as 2 sets of images: contiguous 2.5 or 5 mm collimation and noncontiguous 1.25 mm collimation. The patients were also imaged prone with 1.25 mm axial high resolution images at 2 cm intervals. Studies were reviewed from motion artifact, and radiation doses were calculated for each technique. The axial images had significantly less artifact. The effective radiation dose for the axial technique was lower (.58 mSv vs. 6.17 mSv). The authors advise the addition of several prone axial high-resolution images following volumetric acquisition of data with MDCT. With respect to the study performed by Schoepf (see below), the authors believe that the differences in results are due to the fact that the other study compared studies from different patients.

• Summary: In this study, two sets of subjects underwent either single detector CT (5 mm collimation) and high resolution axial images (1 mm collimation) or 1 mm 4 slice MDCT reconstructed with 1.25 mm and 5 mm slice thickness. Reviewers compared image quality, spatial and contrast resolution, signal-to-noise ratio (subjective), diagnostic value, depiction of the anatomy, motion and streak artifacts. Radiation doses were compared as well. The 5 mm multislice images were rated significantly superior to the 5 mm single detector images. The total scores for high resolution multislice studies were not significantly different from the high resolution single slice acquisitions. The doses were similar (5.55 mSv for MDCT and 5.5 mSv for single detector helical).

• Summary: This review article covers lung anatomy, technology and technique of HRCT, pitfalls, clinical indications and the CT findings in a number of pertinent pathologic conditions.