Purpose / Introduction
Prostate MRI finds 18% more clinically significant prostate cancer while avoiding 27% biopsies [1]. Reproducibility for multi-parametric T2+DWI+DCE
prostate MRI mpMRI is moderate [2] even though a PIRADS reading standard is available [3]. Quantification could help improve reproducibility, which to
some extent works for ADC. Scanner provided T2 maps are no solution as it leads to a different texture, lower spatial resolution and increased scan time.
We have previously developed a method for normalizing T2-weighted images [4]. The normalized value achieved a diagnostic accuracy AUC of 0.85
over 0.64 for the raw T2-weighted values. That method required a separate proton density weighted sequence, an exact knowledge of the sequence
model and one reference tissue region. We propose a new method using multiple reference tissues that does not require an additional sequence, nor
detailed knowledge about the sequence model. The recent development of deep learning makes it feasible to segment multiple reference tissues. The
hypothesis is that the reference tissues allow building a patient specific model to normalize the T2-weighted prostate MR images for quantitative use.
Subjects and Methods
To test the hypothesis we manually delineated reference tissues and tumor lesions in mpMRI studies of prostate cancer patients. All lesions were
interpreted by expert radiologists and assigned a PIRADS score. The normalized T2 was then validated for its ability to discriminate PIRADS 2-3 from 4-5
classes. Regions of interest ROI were drawn in four distinct tissue types in fifty T2-weighted images from regular multiparametric prostate MRI mpMRI.
The four reference tissue types were: obturator internus muscle, body fat, femoral head, bladder lumen. Four average ROI signals were computed per
patient. Each reference tissue was assigned a fixed reference value T2 relaxation found in literature. Per patient, a smooth spline model was fitted to the
average, reference pairs. The estimated spline model was then inverted to map patients' raw T2-weighted image scalar values to normalized values. The
effect of the normalization was determined by computing and comparing the diagnostic accuracy using ROC analysis.
Results
The area under the ROC AUC was significantly higher p<0.05 in normalized T2.5/22/2017 #542: Feasibility of multireference tissue normalization of T2weighted prostate MRI.
Discussion / Conclusion
The significant improvement of the diagnostic accuracy demonstrates the potential of our normalization method for the quantitative interpretation of T2-weighted prostate MRI. The results were similar to our previous method.The method still requires manual delineation of multiple reference tissues