3D reconstruction aids pediatric SPECT

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When ordered subset expectation maximization (OSEM)-3D SPECT is used, patients stand to benefit from a lower radiopharmaceutical dose, being scanned in half the time it would take to undergo standard filtered back projection (FBP) imaging, or a combination of these advantages, according to research published in this month’s issue of Radiology.

“In this study, we investigated the potential to reduce the radiation dose at pediatric renal SPECT while maintaining or even improving image quality,” wrote the authors of a retrospective study conducted at Children’s Hospital Boston and Harvard Medical School in Cambridge, Mass.

The researchers compared two methods of reconstructing technetium 99m (99mTc) dimercaptosuccinic acid (DMSA) renal SPECT data—OSEM-3D and FBP—in children in terms of improving image quality and reducing the radiopharmaceutical activity and radiation dose.

The team retrospectively analyzed 50 sequential pediatric patient 99m Tc-DMSA SPECT studies of 98 kidneys obtained using a dual-detector gamma camera (ECAM, Siemens Healthcare).

“We retrieved all patient SPECT data from our long-term digital image archives for reprocessing and analysis and then reconstructed SPECT image data sets from both camera detectors by using FBP with a fifth-order Butterworth filter at a 0.4 cycle per Nyquist cutoff frequency,” the authors wrote. “Next, we reconstructed the SPECT data from only one of the detectors by using OSEM-3D, as implemented on the gamma camera system (with fast low-angle shot three-dimensional imaging), with use of eight subsets with four iterations and no post-reconstruction filtering.”

Two nuclear medicine physicians blinded to the patients' medical histories and reconstruction techniques evaluated the studies. Each reviewer analyzed the data sets using three-view SPECT display (transverse, sagittal and coronal) images and rotating maximum intensity projection images. Data was also processed to determine the renal length and relative renal function. Scores for image quality, renal size, and relative function were compared by using paired t tests.

Maximum intensity projection 99mTc-DMSA renal SPECT images in 8-month-old boy with Streptococcus durans–related urinary tract infection during first few weeks of life, with bilateral (left grade V, right grade III) reflux. (a) Full-count FBP and (b) half-count OSEM-3D SPECT images show loss of left renal parenchymal function with cortical defects, which are better appreciated in b. R=right. Image and caption courtesy of the Radiological Society of North America.

Image quality was significantly enhanced with OSEM-3D, and cortical defects were identified better on OSEM-3D images than on FBP images, the researchers reported. Of the 98 kidney SPECT studies analyzed, 19 showed identical cortical defects and 75 showed none at both OSEM-3D and FBP. In four kidneys, OSEM-3D depicted cortical defects that were not seen with FBP. No significant difference in relative renal function between the two methods was observed.

“Although the OSEM-3D images were reconstructed with 50 percent fewer gamma photon counts than were the FBP images, the image quality of the OSEM-3D studies (mean score, 4.3 _ 0.7 [standard deviation]) was superior to that of the FBP studies (mean score, 3.5 _ 0.9) (P _ .001),” the authors wrote.

They observed that although no clinically important difference in renal size, differential function, or extent of cortical defects was noted, there was compelling evidence that OSEM-3D reconstruction, as compared with FBP reconstruction, yields superior image quality.

The results indicate that use of this technique has the potential delivering superior image quality for reporting clinicians as well as markedly reduced radiation doses and shorter scanning times for patients.

“Such versatility may pave the way for customizing nuclear medicine examinations to the unique clinical profiles of each patient and, by extension, for improving overall disease management and therapeutic outcomes in the future,” the authors noted.