Multiparametric MRI (mpMRI)

Taken from Chatterjee et al [1]

Prostate mpMRI consists of T2-weighted imaging (T2WI), diffusion weighted imaging (DWI), pre-contrast T1-weighted imaging (T1WI) and dynamic contrasted enhanced (DCE) MRI. mpMRI imaging is highly sensitive in the detection of prostate cancer compared to any individual sequence. A study [2] reported that individually T2 weighted MR (58%) and DWI (53%) and DCE-MRI (38%) imaging sequences demonstrate lower sensitivity compared to mpMRI (85%). Reported positive and negative predictive value for prostate cancer detection using mpMRI is also much higher than that using either of these imaging sequences individually.

T2-Weighted Imaging (T2WI)

T2-weighted imaging allows excellent soft tissue contrast along with good spatial resolution and high signal-to-noise ratio. It provides good visualization of the zonal anatomy, seminal vesicles and neurovascular bundle. High resolution T2WI with 2D turbo/fast spin echo or 3D spin echo sequences with in plane resolution ≤ 0.4-0.7 mm and slice thickness ≤ 3mm is recommended. Images are acquired in the axial, sagittal and coronal planes, while DWI, T1W and DCE-MRI are obtained in the same planes as axial T2WI. Prostate cancer is hypointense on T2-weighted images compared to benign tissue and [2-6]. quantitative T2 values are significantly lower in prostate cancer compared to benign prostate tissue [7-11].

Diffusion-Weighted Imaging (DWI)

Diffusion weighted imaging provides a signal sensitive to water movement and provides information about the tissue structure and density. A spin echo, echo planar imaging pulse sequence with plane resolution > 2.5 mm and slice thickness ≤ 4 mm is generally utilized clinically. Images with at least two b-values are required to acquire an apparent diffusion coefficient (ADC) map using a mono-exponential signal decay model. A lower b-value of 50-100 s/mm2 and higher 800-1500 s/mm2 is generally used. On high b-value (~1500 s/mm2 images) increased signal is seen in prostate cancer due to reduced diffusivity. ADC is a measure of the magnitude of the diffusivity of water molecules and is used extensively clinically for the detection of cancer. ADC in cancer tissue is lower than in normal tissue and there is an inverse relation between ADC value and cancer Gleason grade [12]. A meta-analysis of 10 studies reported that the combined use of diffusion MRI, specifically ADC with traditional T2 weighted images demonstrated higher sensitivity (76%) and specificity (82%) compared to T2 weighted images alone [13]. In addition, tumour volumes estimated from DWI have been shown to demonstrate better correlation with histological volume either than T2W or DCE-MRI [14].

T1-Weighted Imaging (T1W)

Pre-contrast T1W images are taken over a large field of view using spin or gradient echo pulse sequence either with or without fat suppression to observe post biopsy changes. T1 hyperintensity in the prostate is usually due to hemorrhage in the prostate after biopsy.

Dynamic Contrasted Enhanced MRI (DCE-MRI)

DCE-MRI involves the acquisition of serial T1-weighted images (fast/spoiled gradient echo sequence with fat suppression) of the prostate before and after the bolus injection of a chelated gadolinium (Gd) molecule. Cancers show early focal signal enhancement due to increased vascularity or angiogenesis [15, 16]. Increased capillary permeability leads to higher uptake of contrast agent that shortens T1 relaxation time and therefore shows up as hyperintense region with respect to surrounding tissue. Temporal resolution less than 10 seconds is recommended as use of higher temporal resolution leads to increased diagnostic performance [17].

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