Benjamin Patel
In the last decade, magnetic resonance imaging (MRI) has become pivotal in the staging and investigation of urological malignancy and has had a transformative effect on prostate cancer care pathways.
Nuclei, made up of protons and neutrons, are charged particles with a specific motion or ‘precession.’ When a human body is placed in a strong magnetic field, many of the free, randomly aligned hydrogen nuclei align themselves with the direction of the magnetic field. This behaviour is termed Larmor precession. To generate a magnetic resonance (MR) image, a radio‐frequency pulse with a frequency equal to the Larmor frequency is applied perpendicular to the magnetic field, causing the net magnetic moment to tilt away from the direction of the magnetic field. Once the radio‐frequency signal is halted, the nuclei realign themselves with their net magnetic moment parallel to the strong magnetic field. During this ‘relaxation’, the nuclei lose energy and emit their own radiofrequency signal, referred to as the ‘free‐induction decay (FID) response signal.’ The FID response signal can then be measured by a field coil placed around the body being imaged. This measurement can be reconstructed to generate three‐dimensional MR images.
There are two types of relaxation: longitudinal (T1) and transverse (T2). T1 measures the time taken for the magnetic moment of the displaced nuclei to return 63% to thermal equilibrium. Water and cerebrospinal fluid (CSF) have long T1 values, appearing dark on T1 weighted images, whereas fat has a short T1 value and appears bright. T1‐weighted imaging (T1WI) is particularly useful in identifying post‐biopsy haemorrhage and detecting the status of lymph nodes and skeletal metastases, especially in combination with IV gadolinium‐based contrast. T2 on the other hand measures the time required for the FID response signal to decay.
The utility of single sequence T1WI in evaluating the prostate is limited by poor differentiation between prostate and surrounding tissues, artefact from bowel motility and poor intra‐prostatic tissue resolution. Multi‐parametric MRI (mpMRI) aims to obtain an ideal three‐dimensional prostate image by combining T2‐weighted imaging (T2WI), diffusion‐weighted imaging (DWI), and dynamic contrast‐enhanced imaging (DCEI). In general, intestinal motility‐reducing drugs and endorectal coils are used to reduce signal artefact associated with intestinal peristalsis.
T2WI detects the low intensity of neoplastic tissue. Its high resolution provides a sharp demarcation in the prostate capsule. However, in isolation, it is poor at detecting transitional zone and central zone cancers. Diffusion‐weighted imaging provides an ‘apparent diffusion coefficient’ (ADC) map and high b‐value images. Clinically significant cancers appear hypointense in the ADC maps due to restricted diffusion. DWI is better at identifying transitional zone and central zone tumours, as well as cancer aggressiveness, but has poor resolution. DCEI uses gadolinium‐based contrast agent to visualise angiogenesis and thus evaluate the vascularity of tumour.
Prostate Imaging Reporting and Data System (PI‐RADS) was established in 2012 by the European Society of Urogynaecologic Radiology to standardise reporting of prostate MRI and was updated in 2015 with the release of PI‐RADSV2. A score from 1 to 5 is assigned, with 1 indicating that clinically significant cancer is highly unlikely, 3 indicating that clinically significant cancer is equivocal, and 5 indicating that clinically significant cancer is highly likely. Interest in mpMRI has accelerated following publication of PROMIS (Prostate MR Imaging Study), which evaluated the diagnostic accuracy of mpMRI before biopsy and concluded that mpMRI might allow 27% of patients with raised prostate‐specific antigen (PSA) to avoid biopsy.
As detection rates of renal masses continue to increase, clinicians have aimed to improve characterisation of these lesions. The first step is to differentiate benign cysts from solid masses, which contain little or no fluid. This can generally be done with ultrasound, with indeterminate or solid masses then undergoing further characterisation with contrast‐enhanced CT or MRI. The most common solid malignant renal masses are renal cell carcinoma and urothelial carcinoma, whereas the most common solid benign renal masses are angiomyolipoma (AML) and oncocytoma.
Magnetic resonance imaging is a useful imaging tool for diagnosis and characterisation of renal lesions because it provides excellent soft‐tissue contrast. In renal cell carcinoma, a hypointense pseudo capsule may be seen on both T1 and T2‐weighted images. Interruption of this capsule correlates with invasion of perirenal fat. DWI and dynamic contrast enhanced (DCE) can provide further information regarding the tumour histology: there appears to be an inverse relationship between the apparent diffusion coefficient (ADC) value and Fuhrman grade. MRI is thus useful in differentiating benign from malignant lesions as well as predicting the subtype and tumour grade.
Classic angiomyolipomas (AMLs) are identified on MRI because they manifest with the hallmark of bulk fat, providing a high T1 signal. Lipid‐poor AMLs are more difficult to distinguish from renal cell carcinoma (RCC). The typical enhancement pattern is of early intense enhancement with subsequent washout, high signal‐intensity index, and low tumour‐to‐spleen signal‐intensity ratio.
MRI is utilised in the staging of many urological cancers, according to the tumour/node/metastases (TNM) classification.
In prostate cancer, T2WI is fundamental in assessing extra‐capsular extension, seminal vesicle invasion, and lymph node metastasis. Staging accuracy is enhanced using endorectal surface coil and the evolving role of DWI and DCE.
MRI is increasingly used in the staging of bladder cancer to assist in the differentiation of T2 and T3 disease, having been demonstrated to better assess intramural and extravesicular tumour invasion compared with CT. High resolution T2WI of the bladder in three planes with a small field of view and large matrix are used to evaluate the detrusor muscle. Potential artefacts include inappropriate bladder distension, chemical shift, and motion artefact. Optimal bladder distension is achieved by having the patient void two hours before imaging. Bowel peristalsis can be minimised by administrating anti‐motility agents. Chemical shift is reduced by increasing the bandwidth and selecting the frequency‐encoding gradient direction that least interferes with examination of the bladder wall.
Staging of penile cancer can be improved with MRI in combination with induced erection using prostaglandin E1, to exclude tumour invasion of the corpora cavernosa. However, imaging is not a reliable tool for detecting abnormal inguinal nodes. Distant metastases are generally assessed using computerized tomography/proton emission tomography (CT/PET).
MRI has the obvious advantage of not using ionising radiation. It provides excelled contrast between different soft tissues and higher resolution than CT. It can also scan in any plane. However, machines remain significantly more expensive and scans take more time than CT. More artefacts are encountered in MRI. In addition, MRI is contraindicated in patients with internal ferrous objects, such as aneurysm clips. In children, a general anaesthetic may be required. It is also less useful in patients with claustrophobia, due to the enclosed space.