Multiparametric MRI in the Diagnosis of Prostate Cancer: Physical Foundations, Limitations, and Prospective Advances of Diffusion-Weighted MRI

Abstract

Background Diffusion-weighted imaging (DWI) is an essential component of the multiparametric MRI exam for the diagnosis and assessment of prostate cancer (PCa). Over the last two decades, various models have been developed to quantitatively correlate the DWI signal with microstructural characteristics of prostate tissue. The simplest approach (ADC: apparent diffusion coefficient) – currently established as the clinical standard – describes monoexponential decay of the DWI signal. While numerous studies have shown an inverse correlation of ADC values with the Gleason score, the ADC model lacks specificity and is based on water diffusion dynamics that are not true in human tissue. This article aims to explain the biophysical limitations of the standard DWI model and to discuss the potential of more complex, advanced DWI models. Methods This article is a review based on a selective literature review. Results Four phenomenological DWI models are introduced: diffusion tensor imaging, intravoxel incoherent motion, biexponential model, and diffusion kurtosis imaging. Their parameters may potentially improve PCa diagnostics but show varying degrees of statistical significance with respect to the detection and characterization of PCa in current studies. Phenomenological model parameters lack specificity, which has motivated the development of more descriptive tissue models that directly relate microstructural features to the DWI signal. Finally, we present two of such structural models, i. e. the VERDICT (Vascular, Extracellular, and Restricted Diffusion for Cytometry in Tumors) and RSI (Restriction Spectrum Imaging) model. Both have shown promising results in initial studies regarding the characterization and prognosis of PCa. Conclusion Recent developments in DWI techniques promise increasing accuracy and more specific statements about microstructural changes of PCa. However, further studies are necessary to establish a standardized DWI protocol for the diagnosis of PCa. Key Points: Citation Format

Diffusion-Weighted MRI in the Genitourinary System

“…Most of the prostate mpMRI examinations are based on ADC maps provided by only two different b-values and a monoexponential model where tissue diffusion is presumably free with Gaussian distributed distances [ 47 ]. However, more complex diffusion models have been developed to explore the microarchitectural tissue properties [ 48 ].…”

“…However, more complex diffusion models have been developed to explore the microarchitectural tissue properties [ 48 ]. Increasing the amount of coefficient numbers in mathematical models improves the description of the water diffusion in the prostate as demonstrated by statistical methods, such as the Akaike Information Criterion [ 47 , 49 , 50 ]. For example, the addition of a second parameter in the biexponential model better illustrates the difference in diffusion properties due to multiple tissue compartments.…”

“…Such observations led to the biexponential “three-coefficients” model, called the intravoxel incoherent motion (IVIM) model which takes into account the blood flow in the capillary network [ 51 ]. The IVIM model requires many b-values to produce three parametric maps of the perfusion fraction, the pure diffusion and the pseudodiffusion coefficient, respectively [ 47 ]. Pseudodiffusion originates from microscopic circulation in tissue predominant at low b-values (<200 s/mm 2 ) and can be differentiated from the pure diffusion prevailing at higher b-values secondary to Brownian motion within the extravascular space [ 51 ].…”

Diffusion Restriction Comparison between Gleason 4 Fused Glands and Cribriform Glands within Patient Using Whole-Mount Prostate Pathology as Ground Truth

“…Multiparametric magnetic resonance imaging (MP-MRI) has shown promise in improving the diagnostic accuracy of high-grade prostate cancer [ 15 , 16 ]. Apparent diffusion coefficient (ADC) images are often used to identify areas of prostate cancer that restrict diffusion [ 17 ]. Standardization of the prostate imaging reporting and data system (PI-RADS) has also improved the consistency of clinical radiology reads [ 18 , 19 ].…”

Diffusion-Weighted MRI in Patients with Testicular Tumors—Intra- and Interobserver Variability

“…DWI may be used for evaluation of tumors and has shown potential in clinical settings. During the last decade, DWI, including apparent diffusion coefficient (ADC), has been applied in discriminating malignant from benign nodules in a variety of organs [ 1 , 2 , 3 , 4 , 5 ]. In general, adding DWI sequences to existing MR protocols is not a complicated and time comsuming task and can easily be applied to existing protocols.…”