Table 2 Description of biophysical principles and image characteristics of different MRI sequences

T2w

Normal peripheral zone contains relatively high proton density, leading to a homogeneous high signal intensity. Linear, wedge-shaped, or oval low-signal intensity lesions may be present but are considered non-malignant [31].

Normal central gland: variable amounts of intermediate signal intensity, which is often replaced by well-circumscribed hyperplastic nodules of BPH with variable signal intensity [32–34].

Diffusion weighed (DWI)

DWI-MRI measures the Brownian motion of water molecules. Reduced diffusion of water in prostate cancer is attributed to the increased cellularity of malignant lesions, with reduction of the extracellular space and restriction of the motion of a larger portion of water molecules to the intracellular space [35]. The amount of diffusion in tissue is determined quantitatively by the apparent diffusion coefficient (ADC) [32]. Lower ADC values present malignant lesions encoded by lower signal intensity (similar to T2w images). Similar to T2w-images detection of prostate cancer in the peripheral zone is more accurate than in the transitional zone, where the high prevalence of benign prostate hyperplasia (BPH) may lead to reduced ADC values like cancerous tissue. Higher Gleason score is associated with decreased ADC, likely due to the dedifferentiated infiltrative growth of these tumors [36, 37]. DWI-image quality and contrast resolution may be hampered by tumor characteristics (e.g. low to intermediate Gleason score), susceptibility artefacts due to magnetic field inhomogeneity and reduced in-plane resolution [27, 32].

Dynamic contrast enhanced (DCE)

DCE-MRI can visualize increased tumor vascularity in prostate cancer lesions. Prostate cancer lesions typically demonstrate early, rapid and intense enhancement with quick wash-out [38, 39]. Tumor vessels are different from physiological vessels and typically have a higher permeability leading to contrast agent leakage in the extracellular-extravascular space. DCE MR images need to be evaluated by direct visual interpretation of dynamic enhanced T1-weighted images. Potential limitations of DCE-MRI are that it may not reliably differentiate prostatitis in the peripheral zone and transition zone tumors from BPH [32]. Second signal intensity may be analysed using time-signal parameters, e.g. determining peak enhancement or calculation of initial area under the curve in the first 60 seconds (iAUC60) [40].