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ROC and LROC Analyses of the Effects of Lesion Contrast, Size, and Signal-to-Noise Ratio on Detectability in PET Images

Department of Molecular and Medical Pharmacology, Division of Nuclear Medicine and Biophysics, and Department of Radiological Sciences, School of Medicine, University of California, Los Angeles
EC Engineering Consultants, Los Gatos, California

Correspondence: For correspondence or reprints contact: Edward J. Hoffman, PhD, UCLA School of Medicine, B2-086, Center for Health Sciences, 10833 LeConte Ave., Los Angeles, CA 90095-6948.

ABSTRACT

Image quality in PET is typically assessed using measures such as contrast recovery, noise variation, and signal-to-noise ratio (SNR). However, these criteria do not directly reflect performance in the clinical use of the images. Lesion detection is a critical task in the clinical interpretation of many PET studies. A receiver operating characteristic (ROC) study is an accepted method for quantitatively evaluating detection performance with respect to factors that influence image quality. ROC and localization ROC (LROC) analyses were conducted to investigate the effects of lesion contrast, SNR, and size on detectability of hot lesions in PET images. Methods: A thorax phantom was imaged with spheres of 3 sizes simulating lesions (0.45,1.0, and 1.9 mL). The relative activity in the lesions and the total number of counts acquired were each varied by factors of 2 to ascertain the effects of contrast and SNR, respectively. Measured attenuation correction and a standard reconstruction protocol were used. Three nuclear medicine physicians and 6 medical physicists participated as readers, rating each image and indicating the suspected lesion location. The area under the calculated ROC and LROC curves (A_z and A_z,LROC) were used as measures of detection performance. Results: Detection performance was shown to increase from virtually random (A₂ 0.5, A_z,LROC 0.2) to superior (A_z > 0.9, A_z,LROC > 0.9) as lesion contrast was increased by 50% and as lesion SNR was doubled. Detection performance was not seen to vary when comparison was made using image-based measures alone. Conclusion: This study quantitatively shows that moderate increases in the image-based measures of lesion contrast and SNR give a relatively large increase in the task-based measure of lesion detection as measured by ROC and LROC analyses. Thus, techniques that give modest increases in lesion contrast or SNR are expected to improve detection. Results will be useful in evaluating improvement in detection for various reconstruction, acquisition, and data analysis methods that enhance contrast or noise performance.

Key Words: receiver operating characteristic analysis • lesion detection • contrast • signal-to-noise ratio

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