Imablog

The second article again comes from the Journal of Nuclear Medicine and discusses the optimization of PET/CT protocols for large patients, something that is quickly becoming a problem in all imaging modalities. Large patients are the bane of all imaging modalities. Noisy, low contrast x-ray images and low count-density nuclear medicine images from increased attenuation and scatter make for images of marginal diagnostic quality.

Most PET imaging protocols call for a fixed amount of activity to be administered to the patient (typically 5-10 mCi) regardless of the patient’s weight. For thin to normal sized patients, this makes for decent images. For larger patients though, often the result is a noisy image. This is typically compensated for by increasing the imaging time per bed position, which of course increases the total imaging time, the possibility of patient motion artifacts and may not be tolerated well by all patients.

Going to a weight based method (0.21 mCi/kg in the paper) for determining the amount of activity to administer the patient along with a small increase in imaging time per bed position is one method proposed by the authors. The idea behind this is that larger patients get a larger dose (up to 20 mCi), which compensates for photons lost due to increased attenuation. This also means that the imaging time per bed position can be kept the same, or increased by a smaller amount. At the 5 min/bed position time recommended by the authors, this comes out to roughly 30-40 minutes/study.

Benjamin S. Halpern, MD, Magnus Dahlbom, PhD, Martin A. Auerbach, MD, Christiaan Schiepers, MD, PhD, Barbara J. Fueger, MD, Wolfgang A. Weber, MD, Daniel H.S. Silverman, MD, PhD, Osman Ratib, MD, PhD and Johannes Czernin, MD, “Optimizing Imaging Protocols for Overweight and Obese Patients: A Lutetium Orthosilicate PET/CT Study“, JNM 46: 603-607

Abstract:

High photon attenuation and scatter in obese patients affect image quality. The purpose of the current study was to optimize lutetium orthosilicate (LSO) PET image acquisition protocols in patients weighing ≥91 kg (200 lb).
Methods: Twenty-five consecutive patients (16 male and 9 female) weighing ≥91 kg (200 lb; range, 91-168 kg [200-370 lb]) were studied with LSO PET/CT. After intravenous injection of 7.77 MBq (0.21 mCi) of ¹⁸F-FDG per kilogram of body weight, PET emission scans were acquired for 7 min/bed position. Single-minute frames were extracted from the 7 min/bed position scans to reconstruct 1-7 min/bed position scans for each patient. Three reviewers independently analyzed all 7 reconstructed whole-body images of each patient. A consensus reading followed in cases of disagreement. Thus, 175 whole-body scans (7 per patient) were analyzed for number of hypermetabolic lesions. A region-of-interest approach was used to obtain a quantitative estimate of image quality. Results: Fifty-nine hypermetabolic lesions identified on 7 min/bed position scans served as the reference standard. Interobserver concordance increased from 64% for 1 min/bed position scans to 70% for 3 min/bed position scans and 78% for 4 min/bed position scans. Concordance rates did not change for longer imaging durations. Region-of-interest analysis revealed that image noise decreased from 21% for 1 min/bed position scans to 14%, 13%, and 11% for, respectively, 4, 5, and 7 min/bed position scans. When compared with the reference standard, 14 lesions (24%) were missed on 1 min/bed position scans but only 2 (3%) on 4 min/bed position scans. Five minute/bed position scans were sufficient to detect all lesions identified on the 7 min/bed position scans. Conclusion: Lesion detectability and reader concordance peaked for 5 min/bed position scans, with no further diagnostic gain achieved by lengthening the duration of PET emission scanning. Thus, 5 min/bed position scans are sufficient for optimal lesion detection with LSO PET/CT in obese patients.