Practical issues and limitations of brain attenuation correction on a simultaneous PET-MR scanner

J E Mackewn,K Beck,O Howes,S Jeljeli,R I Johnstone,P K Marsden,J Stirling,L C Pike,C J Mcginnity,A Hammers,I Merida,S-M Gould

doi:10.1186/s40658-020-00295-x

Abstract

BackgroundDespite the advent of clinical PET-MR imaging for routine use in 2011 and the development of several methods to address the problem of attenuation correction, some challenges remain. We have identified and investigated several issues that might affect the reliability and accuracy of current attenuation correction methods when these are implemented for clinical and research studies of the brain. These are (1) the accuracy of converting CT Hounsfield units, obtained from an independently acquired CT scan, to 511 keV linear attenuation coefficients; (2) the effect of padding used in the MR head coil; (3) the presence of close-packed hair; (4) the effect of headphones. For each of these, we have examined the effect on reconstructed PET images and evaluated practical mitigating measures.ResultsOur major findings were (1) for both Siemens and GE PET-MR systems, CT data from either a Siemens or a GE PET-CT scanner may be used, provided the conversion to 511 keV μ-map is performed by the PET-MR vendor’s own method, as implemented on their PET-CT scanner; (2) the effect of the head coil pads is minimal; (3) the effect of dense hair in the field of view is marked (> 10% error in reconstructed PET images); and (4) using headphones and not including them in the attenuation map causes significant errors in reconstructed PET images, but the risk of scanning without them may be acceptable following sound level measurements.ConclusionsIt is important that the limitations of attenuation correction in PET-MR are considered when designing research and clinical PET-MR protocols in order to enable accurate quantification of brain PET scans. Whilst the effect of pads is not significant, dense hair, the use of headphones and the use of an independently acquired CT-scan can all lead to non-negligible effects on PET quantification. Although seemingly trivial, these effects add complications to setting up protocols for clinical and research PET-MR studies that do not occur with PET-CT. In the absence of more sophisticated PET-MR brain attenuation correction, the effect of all of the issues above can be minimised if the pragmatic approaches presented in this work are followed.

Highlights

Despite the advent of clinical PET-MR imaging for routine use in 2011 and the development of several methods to address the problem of attenuation correction, some challenges remain
In addition we provide measurements of sound levels that demonstrate under most circumstances it is safe to scan without headphones, eliminating the problem altogether
The Hounsfield units (HUs) measured for the six samples labelled with their density to the immediate right, in the Fig. 4a, were higher for the GE image compared to the Siemens image by an average 3.7%

Summary

Introduction

We have identified and investigated several issues that might affect the reliability and accuracy of current attenuation correction methods when these are implemented for clinical and research studies of the brain. These are (1) the accuracy of converting CT Hounsfield units, obtained from an independently acquired CT scan, to 511 keV linear attenuation coefficients; (2) the effect of padding used in the MR head coil; (3) the presence of close-packed hair; (4) the effect of headphones. Current techniques that have been fairly widely used for brain attenuation correction are as follows: 1) Using a CT scan measured independently on a PET-CT scanner This requires the development of a process to import a registered CT-derived μ-map into the PETMR reconstruction pipeline

Methods

Results

Discussion

Conclusion