Magnetic resonance imaging of the prostate.

MRI of the prostate has been used since the mid-1980s, but the information content and accuracy of this method were limited for a long time due to the technical imperfection of MRI scanners and the insufficient development of the examination methodology.

The outdated name of the method - nuclear magnetic resonance imaging (NMR) - is no longer used to avoid incorrect associations with ionizing radiation.

The purpose of performing MRI of the prostate

The main purpose of MRI of the pelvis is local and regional staging of the oncological process according to the TNM system.

Basic principles of magnetic resonance imaging

MRI is based on the phenomenon of nuclear magnetic resonance, discovered in 1946 by physicists F. Bloch and E. Purcell (Nobel Prize in Physics, 1952). This phenomenon consists of the ability of nuclei of some elements, under the influence of a static magnetic field, to receive the energy of a radiofrequency pulse. Parallel work on the study of electron paramagnetic resonance was carried out at the Kazan State University by Professor E.K. Zavoisky. In 1973, the American scientist P. Lauterbur proposed to supplement the phenomenon of nuclear magnetic resonance with the effect of an alternating magnetic field to determine the spatial location of the signal. Using the image reconstruction technique, which was used at that time in CT, he managed to obtain the first MRI of a living being. In 2003, P. Lauterbur and P. Mansfield (the creator of ultra-fast MRI with the ability to obtain one image in 50 ms) were awarded the Nobel Prize in Physiology or Medicine. Today, there are more than 25 thousand MRI scanners in the world, which perform more than half a million studies per day.

The most important advantage of MRI compared to other diagnostic methods is the absence of ionizing radiation and, as a consequence, the complete elimination of the effects of carcinogenesis and mutagenesis.

Advantages of magnetic resonance imaging:

• high spatial resolution;
• absence of ionizing radiation, carcinogenesis and mutagenesis effects;
• high soft tissue contrast;
• the ability to accurately detect infiltration and tissue swelling;
• the possibility of tomography in any plane.

MRI has high soft tissue contrast and allows for examination in any plane, taking into account the anatomical features of the patient's body, and, if necessary, to obtain three-dimensional images for an accurate assessment of the prevalence of the pathological process. Moreover, MRI is the only non-invasive diagnostic method that has high sensitivity and specificity in detecting edema and infiltration of any tissue, including bone.

The main technical parameter of MRI is magnetic field strength, which is measured in Tesla (T). High-field tomographs (from 1.0 to 3.0 T) allow the widest range of studies of all areas of the human body, including functional studies, angiography, and rapid tomography. Low- and medium-field tomography (less than 1.0 T) does not provide clinically significant information about the state of the prostate gland. In the last 2-3 years, MRI tomographs with a magnetic field strength of 3.0 T have become of greatest interest and have become available for full-fledged clinical use. Their main advantages are the ability to obtain images with high spatial resolution (less than 1 mm), high speed, and sensitivity to minimal pathological changes.

Another important technical factor determining the informativeness of MRI in pelvic examinations is the type of RF sensor, or coil, used. Phased RF coils for the body are usually used, which are placed around the examination area (one element at the lumbar level, the second on the anterior abdominal wall). Endorectal sensors have significantly expanded the diagnostic capabilities of MRI due to a significant increase in spatial resolution and signal-to-noise ratio in the examination area, clear visualization of the prostate capsule and neurovascular bundles. Currently, work is underway to create endorectal sensors for MRI scanners with a magnetic field strength of 3.0 T.

The accuracy of MRI diagnostics and the characteristics of hypervascular processes (tumors, inflammation) can be significantly increased by using artificial contrast.

With the advent of specialized endorectal sensors (radiofrequency coils), dynamic contrast and spectroscopy, MRI quickly attracted the attention of many clinicians and researchers and gradually entered the range of diagnostic examinations of patients with prostate cancer. The slow development of this area of radiological diagnostics in our country was due to the insufficient prevalence of radical methods of treating prostate cancer (including prostatectomy and radiation therapy), low availability of modern tomographs and the lack of appropriate training programs for specialists in radiation diagnostics and urologists. In recent years, the situation has begun to change for the better against the background of increased government purchases of medical equipment and the emergence of specialized centers for the diagnosis and treatment of prostate cancer.

Indications for the procedure

The main indications for magnetic resonance imaging in patients with prostate cancer are:

• differentiation of stages T2 and T3 to determine indications for surgical or radiation treatment in patients with medium and high risk of extraprostatic tumor spread;
• assessment of the condition of regional lymph nodes and detection of metastases in the bones of the pelvis and lumbar spine (more accurate diagnostics compared to CT);
• tumor differentiation grade according to Gleason is more than 6;
• stage T2b according to digital rectal examination;
• assessment of the dynamics of the state of the prostate gland, lymph nodes and surrounding tissues in patients with continued growth of prostate cancer against the background of treatment;
• detection of local recurrences of prostate cancer or metastases to regional lymph nodes in case of biochemical recurrence of cancer after radical prostatectomy;
• PSA level >10 ng/ml.

When formulating indications for MRI, it is necessary to take into account the dependence of the accuracy of this method on the presence of locally advanced prostate cancer, determined by the PSA level and the degree of tumor differentiation.

Diagnostic efficacy of magnetic resonance imaging depending on the presence of locally advanced prostate cancer

	Low risk (PSA <10 ng/ml, Gleason 2-5)	Average squeak (PSA = 10-20 ng/ml, Gleason 5-7)	High risk (PSA>20 ng/ml, Gleason 8 10)
Tumor detection	Low	Tall	Tall
Determination of local prevalence	Tall	Tall	Tall
Detection of lymphadenopathy	Average	Average	Tall

In addition, magnetic resonance imaging of the prostate gland is performed to clarify the characteristics of cystic prostatic and periprostatic structures, identify complications of prostatitis and the characteristics of prostate adenoma.

Patients with negative results of repeated biopsies (more than two) in the anamnesis, PSA level within the “gray scale” (4-10 ng/ml), absence of pathology in TRUS and digital rectal examination are recommended to undergo MR planning of biopsy, during which areas suspicious for the presence of a neoplastic process are identified.

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Preparation

In patients with suspected prostate cancer, MRI of the pelvis can be performed both before transrectal biopsy (if there is a serum PSA result) and 3-4 weeks after it (after the disappearance of areas of post-biopsy hemorrhage in the prostate gland). The study should be performed on a high-field tomograph (at least 1 T), if possible - with an endorectal sensor, in at least two perpendicular planes using dynamic contrast.

Preparation for MRI examination of the prostate gland (endorectal and superficial) consists of cleansing the rectum with a small enema. The examination is carried out with a full bladder, if possible - after suppression of peristalsis with intravenous administration of glucagon or giosnip butyl bromide.

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Technique MRI of the prostate

The endorectal sensor is installed at the level of the prostate gland and filled with air (80-100 ml), which ensures clear visualization of the prostate capsule, rectoprostatic angles and rectoprostatic fascia. The use of an endorectal sensor does not limit the ability to visualize regional lymph nodes (up to the level of the bifurcation of the abdominal aorta), since the study is carried out using a combination of pelvic (external) and endorectal (internal) coils.

The patient is placed inside the tomograph in a supine position. The examination begins with fast tomography (localizer) to control the location of the sensor and plan subsequent programs. Then T2-weighted images are obtained in the sagittal plane to assess the general anatomy of the pelvis. T1-weighted images in the axial plane are used to assess lymphadenopathy zones, detect blood in the prostate and metastases in the pelvic bones. Targeted axial T2-weighted tomograms with a slice thickness of about 3 mm are the most informative for assessing the prostate gland. Fast tomography with T1-weighted images and signal suppression from fatty tissue is used to perform dynamic contrasting of the prostate gland and assess the lymph nodes. The total duration of the examination is about 25-30 minutes.

Protocol of endorectal magnetic resonance imaging in prostate cancer

Pulse sequence	Plane	Slice thickness/interval, mm	Task
T2-VI (spin echo)	SP	5/1	Assessment of the general anatomy of the pelvic organs
T1-VI (spin echo)	AP (through the pelvis)	5/1	Search for lymphadenopathy, evaluation of pelvic bones
T2-WI (spin echo) targeting the prostate gland	AP	3/0	Evaluation of the prostate gland and seminal vesicles
	Kp/sp	3/0	Evaluation of the prostate gland and seminal vesicles
T1-WI (gradient echo) with fat suppression, intravenous contrast and multiphase scanning	AP	(1-3)/0	Evaluation of the prostate gland and seminal vesicles

Notes: SP - sagittal plane; AP - axial plane; CP - coronal plane; VI - weighted image.

Scanning is performed without holding the breath. When performing tomography in the axial plane, it is necessary to use the transverse direction of phase encoding (from left to right) in the fields to reduce the severity of artifacts from vascular pulsation and movement of the anterior abdominal wall. Also, it is possible to use presaturation of the anterior abdominal wall area. Processing of the obtained images should include a program for correcting the intensity of the signal of the surface coil (BOS), which ensures a uniform signal from the entire pelvic area, and not only from the prostate gland.

Of the MR contrast agents, 0.5 M contrast agents (GD-DTPA) are usually used at a rate of 0.1 mmol, or 0.2 ml, per 1 kg of the patient's body weight (the volume of the contrast agent usually does not exceed 15-20 ml per study). When conducting MR studies with dynamic multiphase contrast, it is preferable to use 1.0 M agents (gadobutrol), since with a smaller injection volume (7.5-10 ml) compared to 0.5 M agents, it is possible to achieve a more optimal bolus geometry, thereby increasing the information content of the arterial phase of contrast.

Contraindications to the procedure

Contraindications to MRI are associated with exposure to magnetic fields and radiofrequency (non-ionizing) radiation.

Absolute contraindications:

• artificial pacemaker;
• intracranial ferromagnetic hemostatic clips;
• intraorbital ferromagnetic foreign bodies;
• middle or inner ear implants;
• insulin pumps;
• neurostimulators.

Most modern medical devices installed in the patient's body are conditionally compatible with MRI. This means that examination of patients with installed coronary stents, intravascular coils, filters, and heart valve prostheses can be performed if clinically indicated in agreement with a specialist in radiation diagnostics based on the manufacturer's information on the characteristics of the metal from which the installed device is made. If there are surgical materials and instruments with minimal magnetic properties (some stents and filters) inside the patient's body, MRI can be performed at least 6-8 weeks after surgery, when fibrous scar tissue will ensure reliable fixation of the device.

Epirectal MRI is also contraindicated for 2-3 weeks after multifocal transrectal prostate biopsy, for 1-2 months after surgical interventions in the anorectal area and in patients with severe hemorrhoids.

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Normal performance

MRI of the pelvic organs includes visualization of the zonal anatomy of the prostate gland, its capsule, seminal vesicles, surrounding tissues, urinary bladder, base of the penis, rectum, gastric bones, regional lymph nodes.

Normal MRI anatomy of the prostate gland

The zonal anatomy of the prostate gland is assessed on T2-weighted images: the peripheral zone is hyperintense, the central zone is iso- or hypointense compared to muscle tissue.

The pseudocapsule of the prostate gland is visualized as a thin hypointense border, which merges with the fibromuscular stroma along its anterior surface. On T1-weighted images, the zonal anatomy of the prostate gland is not differentiated.

The size and volume of the prostate gland are estimated using the formula:

V (mm ³ or ml) = x • y • z • 0.1

The rectoprostatic angles should be free, not obliterated. The rectoprostatic fascia between the prostate gland and the rectum is usually clearly visible on axial tomograms. Neurovascular bundles should be visible on both sides of the posterolateral surface of the prostate gland. The dorsal venous complex is visible on its anterior surface, usually hyperintense on T2-weighted images due to slow blood flow. The seminal vesicles are visualized as fluid cavities (hyperintense on T2-weighted images) with thin walls.

When examined with dynamic contrast, the contents of the vesicles do not accumulate the drug. The membranous part of the urethra is visualized on sagittal or frontal T2-weighted tomograms.

Normal lymph nodes are best seen on T1-weighted images against a background of fat. As with MSCT, node size is the main indicator of metastatic disease.

Normal bone tissue on T1- and T2-weighted images is hyperintense due to the high content of adipose tissue in the bone marrow. The presence of hypointense foci (in the bones of the abdomen, spine, femurs) most often indicates metastatic osteoblastic lesions.

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Benign prostatic hyperplasia

MR signs of the disease depend on the predominant component; glandular hyperplasia is hyperintense on T2-weighted images (with the formation of cystic changes), stromal hyperplasia is hypointense. Against the background of stromal hyperplasia of the prostate gland, it is most difficult to detect cancer of its central parts. The peripheral zone in a large adenoma is compressed, which also complicates the detection of cancer. In a very large adenoma, the peripheral zone can be so compressed that it forms a surgical capsule of the prostate.

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Prostatitis

The basis for prostatitis diagnostics is a clinical examination combined with microbiological studies. If complications are suspected (abscess formation), as well as patients with pelvic pain of unclear etiology, ultrasound or MRI is usually performed. Hypointense lesions in the peripheral zone of the prostate gland on T1-weighted images may correspond to both inflammatory changes and neoplastic lesions, MRI criteria for prostatitis lesions are a cone-shaped hypointense lesions, clear contours, and no mass effect.

Prostate cysts

Cystic changes in the central zone of the prostate gland may occur with its benign hyperplasia (glandular form); retention or post-inflammatory cysts usually occur in the peripheral zone. Congenital prostatic or periprostatic cysts may be combined with other developmental anomalies and may cause infertility, requiring diagnosis and appropriate treatment. Congenital cysts may have different localizations, the most informative method of determining which is MRI.

Most often, intraprostatic cysts originate from the prostatic utricle or vas deferens, while extraprostatic cysts originate from the seminal vesicles and the remnant of the Müllerian duct.

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Adenocarcinoma of the prostate gland

Prostate adenocarcinoma is characterized by low signal intensity on T1-weighted images in the presence of high signal intensity from the normal peripheral zone of the prostate gland.

The most important advantage of endorectal MRI is the ability to accurately localize foci of neoplastic lesions, determine the nature and direction of tumor growth. In particular, MRI allows identifying cancer foci in the anterior sections of the peripheral zone of the prostate gland, which are difficult to access with transrectal biopsy. Irregular shape, diffuse spread with mass effect, unclear and uneven contours are morphological signs of foci of low signal intensity in the peripheral zone of the prostate gland, suggesting a malignant nature of the lesion.

With dynamic contrasting, cancer foci quickly accumulate contrast agent in the arterial phase and quickly remove it, which reflects the degree of neohistogenesis and, accordingly, the degree of tumor malignancy.

Representatives of the North American school of radiology advocate the use of MR spectroscopy instead of dynamic contrast, which is preferred by representatives of the European school of radiology, for the precise localization of cancer foci. This is due, in particular, to the fact that only MR spectroscopy allows non-invasive detection of tumor foci not only in the peripheral, but also in the central zone of the prostate gland.

Endorectal MRI allows direct visualization of the prostate capsule and determination of the local extent of the tumor.

The main criteria for extraorgan spread of prostate cancer (according to MRI data):

• asymmetry of neurovascular bundles;
• obliteration of the rectoprostatic angle;
• bulging of the gland contour;
• extracapsular tumor;
• wide contact of the tumor with the capsule;
• asymmetrically hypointense signal from the contents of the seminal vesicle.

Comparative characteristics of MR criteria for extraprostatic spread of cancer

MR criterion	Accuracy, %	Sensitivity, %	Specificity, %
Asymmetries of neurovascular bundles	70	38	95
Obliteration of the recto-prostatic angle	71	50	88
Bulging of the capsule	72	46	79
Extracapsular tumor	73	15	90
General impression	71	63	72

Severe extracapsular invasion according to MRI data not only determines the inappropriateness of surgical treatment, it is considered an unfavorable prognostic factor.

Pathways of seminal vesicle involvement in prostate cancer:

• tumor growth along the vas deferens;
• direct involvement of vesicles by peripheral tumor;
• a tumor of the bladder not associated with a primary lesion of the prostate gland.

The main signs of seminal vesicle invasion:

• absence of hyperintense signal from the contents on T2-weighted images;
• asymmetric enlargement, hemorrhage into the vesicle.

Hypointense foci in the seminal vesicles may be associated with post-biopsy hemorrhage, amyloidosis (about 30% of men over 75 years of age), and compression by prostate adenoma.

When a prostate tumor spreads to the bladder or rectum, there is no fatty tissue between them.

A study with intravenous contrast allows for a more precise determination of the tumor boundaries.

Hormonal ablation in prostate cancer leads to a decrease in the intensity of the MR signal, a decrease in the size of the gland, which somewhat complicates diagnostics. However, there is no reliable decrease in the accuracy of MR staging against the background of hormonal ablation.

Recently, MRI has increasingly attracted the attention of specialists as a method for planning treatment measures (in particular, radiation therapy and surgical interventions), since modern treatment methods in many cases allow a patient to be cured of an oncological disease, and the issue of their quality of life after treatment comes to the forefront. For this reason, radiation therapy for prostate cancer is carried out after marking the radiation exposure field according to CT or MRI data, which allows protecting adjacent unaffected organs (for example, the neck of the bladder).

MRI before radical prostatectomy allows evaluation of the membranous urethra, the length of which inversely correlates with the severity of urinary dysfunction after surgery. In addition, the severity of the dorsal complex, a potential source of massive bleeding when it is crossed during surgery, is assessed.

It is extremely important to assess the integrity of the neurovascular bundles, along which prostate cancer spreads in most cases. The absence of neurovascular bundle invasion gives hope for the preservation of erectile function after surgery (nerve-sparing surgery). It is also necessary to determine the degree of extraprostatic tumor spread (in millimeters along two axes), since local infiltration of the capsule and periprostatic tissues in patients with highly differentiated tumors is not considered a contraindication for radical prostatectomy.

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Low signal intensity prostate diseases

Low signal intensity is also characteristic of inflammatory changes, especially chronic prostatitis, fibrous-cicatricial changes, fibromuscular or stromal hyperplasia, consequences of hormonal or radiation therapy. MRI without dynamic contrast does not allow reliable differentiation of most of the listed changes and diseases.

Post-biopsy changes in the prostate gland. Characteristic features include unevenness of the prostate capsule, hemorrhages, and changes in the MR signal of the parenchyma.

A full MRI examination becomes possible only after the hemorrhages have disappeared, which on average takes 4-6 weeks (sometimes 2-3 months).

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Operational characteristics of prostate MRI

The average sensitivity of MRI in detecting prostate cancer (primarily microscopic lesions) does not allow this method to be used to exclude a neoplastic process.

In case of biochemical recurrence of cancer after radical prostatectomy, MRI allows to detect local recurrence of the tumor or metastases to regional lymph nodes with 97-100% accuracy.

The accuracy of MRI in detecting foci of neoplastic lesions of the prostate gland is 50-90%. The sensitivity of MRI in localizing prostate cancer is about 70-80%, while microscopic foci of cancer cannot be detected using MRI. Hyperintensity on T2-weighted images of mucinous adenocarcinoma of the prostate gland complicates diagnosis and leads to false-negative MRI results.

Clinical information (PSA level, previous treatment), knowledge of the anatomy of the prostate gland, the use of an endorectal sensor, dynamic contrast and spectroscopy make it possible to bring the accuracy of detecting cancer foci using MRI closer to 90-95% (specificity increases to a greater extent).

The sensitivity of MRI for extraprostatic extension is within 43-87%, which is primarily due to the inability to visualize microscopic invasion of the prostate capsule. The sensitivity of detecting extensions less than 1 mm deep with endorectal MRI is only 14%, while with tumor invasion beyond the gland by more than 1 mm, the figure increases to 71%. In the low-risk group (PSA <10 ng/ml, Gleason score <5), the frequency of detecting tumor spread beyond the prostate gland is low, macroscopic extension is observed quite rarely, which significantly increases the frequency of false-negative results. The sensitivity of detecting seminal vesicle invasion is 70-76%. The highest specificity (up to 95-98%) and prognostic value of a positive MRI result are achieved when examining patients with medium or high risk of extracapsular invasion (PSA>10 ng/ml, Gleason score 7 points or more).

Factors influencing the result

One of the main problems in detecting cancer foci and extracapsular tumor spread is the high variability of tomogram interpretation by different specialists. MRI can only provide reliable results when tomograms are analyzed by qualified specialists in radiation diagnostics with extensive experience in urogenital radiology. Supplementing standard MRI with dynamic contrast enhancement allows for greater standardization of the study and increased accuracy in detecting extracapsular invasion. The main task of a specialist in radiation diagnostics is to achieve high specificity of MRI diagnostics (even at the expense of sensitivity), so as not to deprive operable patients of the chance for radical treatment.

Limitations of prostate magnetic resonance imaging:

• low sensitivity to microscopic lesions;
• false negative results due to the presence of blood in the peripheral zone after biopsy;
• transition of prostate adenoma to the peripheral zone;
• detection of cancer in the central zone of the prostate;
• pseudo-foci in the area of the base of the gland;
• high dependence of diagnostic accuracy on the experience of the radiologist.

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Complications after the procedure

In the vast majority of cases, patients tolerate endorectal MRI examination well. Complications are extremely rare (small bleeding if the patient has defects in the rectal mucosa).

Side effects when using MR contrast agents are extremely rare (less than 1% of cases) and are usually mild (nausea, headache, burning at the injection site, paresthesia, dizziness, rash).

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Prospects for Magnetic Resonance Imaging of the Prostate

Due to the constant improvement of both technical capabilities and diagnostic methods, MRI of the prostate is currently a highly effective method for diagnosing malignant neoplasms of the prostate gland. However, high accuracy of staging of prostate cancer using MRI can only be achieved by using a multidisciplinary approach in clinical work based on the constant interaction of urologists, specialists in radiation diagnostics and pathologists.

A significant diagnostic limitation of both CT and MRI is the low accuracy in diagnosing metastatic lesions of the lymph nodes in the absence of their quantitative and qualitative increase. The main hopes for solving this problem are associated with the development of molecular diagnostics and the creation of lymphotropic contrast agents (currently undergoing clinical trials of phases II-III). As radiation diagnostics develops, spectroscopy, tumoritron and lymphotropic contrast agents begin to be used in clinical practice, MRI may become the most informative complex method for diagnosing prostate cancer, mandatory for patients in the medium and high risk groups, before biopsy or the start of treatment.