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.
Get full access to this article
View all access options for this article.