Impact of Scatter and Attenuation Corrections for Iodine-131 Two-Dimensional Quantitative Imaging in Patients

Abstract

This study assessed the impact of scatter and attenuation corrections on the estimated activity delivered to whole body and liver in five patients included in a radioimmunotherapy clinical trial.

Before injection of the radiopharmaceutical, transmission images were acquired with the Transmission Attenuation Correction - Whole-body (SMVi-GEMS) prototype. Emission images were obtained in energy-indexed list mode at least four times after injection. 20% window and scatter-corrected images (Dual Energy Window-DEW and Triple Energy Window-TEW) were generated.

Whole-body activity was calculated 1-h after injection(and compared with injected activity). Cumulated activities in whole body and liver were determined according to the geometric mean approach.

The mean relative error made in estimations of whole-body activity at 1-h was 6.9+/-10.3% without corrections. Taking scatter into account led to underestimation, but reduced the influence of patient morphotype (-40.0+/-7.6% and -43.3+/-6.2% for DEW and TEW). Attenuation correction led to a large overestimation, whether used alone (155.2+/-39.0%) or associated with scatter correction (39.6+/-10.4% and 35.9+/-10.2% for DEW and TEW).

Compared to the geometric mean alone, scatter correction led to a reduction of cumulated activities of around 45% for whole body and less than 30% for liver. Attenuation correction had a more marked impact, particularly for liver where estimated cumulated activity increased from 150 to 300%. Preliminary scatter correction limited the increase to 100% for DEW and 150% for TEW in liver and to 25% for both DEW and TEW in whole body.

Although this would probably be different at the organ level, the calculation of whole-body activity without scatter and attenuation correction gave the lowest biases. But from a scientific point of view, this cannot be a satisfactory method. Attenuation correction has a greater impact than scatter correction. The association of both corrections is not sufficient to obtain accurate absolute quantification. Other factors limit planar quantification with iodine-131, notably auto-absorption of sources, septal penetration of high-energy photons through the collimator and superimposition of sources.