Improved quantitation and reproducibility in multi-PET/CT lung studies by combining CT information

Beverley F Holman,Brian F Hutton,Toby M Maher,Vesna Cuplov,Lynn Millner,Kris Thielemans,Raymond Endozo,Ashley M Groves

doi:10.1186/s40658-018-0212-0

Abstract

BackgroundMatched attenuation maps are vital for obtaining accurate and reproducible kinetic and static parameter estimates from PET data. With increased interest in PET/CT imaging of diffuse lung diseases for assessing disease progression and treatment effectiveness, understanding the extent of the effect of respiratory motion and establishing methods for correction are becoming more important. In a previous study, we have shown that using the wrong attenuation map leads to large errors due to density mismatches in the lung, especially in dynamic PET scans. Here, we extend this work to the case where the study is sub-divided into several scans, e.g. for patient comfort, each with its own CT (cine-CT and ‘snap shot’ CT). A method to combine multi-CT information into a combined-CT has then been developed, which averages the CT information from each study section to produce composite CT images with the lung density more representative of that in the PET data. This combined-CT was applied to nine patients with idiopathic pulmonary fibrosis, imaged with dynamic 18F-FDG PET/CT to determine the improvement in the precision of the parameter estimates.ResultsUsing XCAT simulations, errors in the influx rate constant were found to be as high as 60% in multi-PET/CT studies. Analysis of patient data identified displacements between study sections in the time activity curves, which led to an average standard error in the estimates of the influx rate constant of 53% with conventional methods. This reduced to within 5% after use of combined-CTs for attenuation correction of the study sections.ConclusionsUse of combined-CTs to reconstruct the sections of a multi-PET/CT study, as opposed to using the individually acquired CTs at each study stage, produces more precise parameter estimates and may improve discrimination between diseased and normal lung.

Highlights

Matched attenuation maps are vital for obtaining accurate and reproducible kinetic and static parameter estimates from PET data
Respiratory attenuation correction (AC) map mismatches are due to two main contributions; motion, which leads to anatomical location mismatch, and density variations due to lung fractional air volume changes over the course of the breathing cycle [10], which could lead to attenuation coefficient mismatch
As Patlak-Rutland only uses the final two study sections, the values of KiP are not affected by the initial scan section, provided the image-derived input function (IDIF) is minimally affected by the early variations in density

Summary

Introduction

Matched attenuation maps are vital for obtaining accurate and reproducible kinetic and static parameter estimates from PET data. PET/CT is becoming more popular for use in diffuse lung diseases such as chronic obstructive pulmonary disease (COPD), interstitial lung disease (ILD), infection and inflammation [1] These studies commonly address disease progression and treatment effectiveness and make use of quantitative dynamic as well as static imaging [2,3,4,5,6]. Quantitative PET images can only be obtained with accurate attenuation correction (AC) maps These AC maps are commonly derived from short CT acquisitions (i.e. a standard helical CT for AC), which are appropriate in regions where internal anatomical motion is unlikely [7, 8]. Mismatches due to motion have been widely explored in the literature [11, 12] and density variations due to the respiratory phase at the time of acquisition have long been acknowledged by the CT community [13, 14], the effect on PET quantitation in the lung has only very recently been acknowledged [15]

Methods

Results

Discussion

Conclusion