Magnetic resonance imaging of the prostate

MRI of the prostate has been used since the mid-80's. XX century. However, the informativeness and accuracy of this method have for a long time been limited due to the technical imperfection of MR scanners and the insufficiently developed method of examination.

The obsolete name of the method - nuclear magnetic resonance (NMR) tomography - is not currently used to avoid incorrect associations with ionizing radiation.

Purpose of MRI of the prostate

The main goal of MRI of the small pelvis is local and regional staging of the oncological process using the TNM system.

The basic principles of magnetic resonance imaging

The 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 is the ability of the nuclei of certain elements under the influence of a static magnetic field to receive the energy of a radio-frequency 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 action of an alternating magnetic field to determine the spatial arrangement of the signal. Using the technique of image reconstruction, which was used at that time for CT, he managed to get the first MR-tomogram of a living creature. In 2003, P. Loterbur and P. Mansfield (creator of ultrafast MRI with the possibility of obtaining one image in 50 ms) were awarded the Nobel Prize in Physiology or Medicine. Today, there are more than 25 thousand MR-tomographs in the world, with a total of more than half a million studies a day.

The most important advantage of MRI in comparison with other diagnostic methods is the absence of ionizing radiation and, as a consequence, the complete elimination of the effects of cancer and mutagenesis.

Advantages of magnetic resonance imaging:

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

MRI has a high soft tissue contrast and allows you to conduct research 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 tissues, including bone.

The main technical parameter of MRI is the magnetic field strength, which is measured in Tesla (T). High-field scanners (from 1.0 to 3.0 T) make it possible to conduct the widest range of studies of all areas of the human body, including functional research, angiography, rapid tomography. Low- and mid-field tomography (less than 1.0 T) does not allow obtaining clinically relevant information about the state of the prostate. In the last 2-3 years, the most interesting are MR tomographs with a magnetic field strength of 3.0 T, which are available for a full clinical application. Their main advantages are the possibility of obtaining images with high spatial resolution (less than 1 mm), high speed, and sensitivity to minimal pathological changes.

Another important technical factor that determines the informativeness of MRI in pelvic examinations is the type of RF sensor used, or coils. Usually, phased RF coils for the body are used, which are placed around the study area (one element at the waist level, the second one at the front abdominal wall). Endorectal sensors significantly increased the diagnostic capabilities of MRI due to a significant increase in spatial resolution and signal-to-noise ratio in the study area, a clear visualization of the capsule of the prostate and neurovascular bundles. Currently, work is under way to create endorektalnyh sensors for MR-tomographs with a magnetic field strength of 3.0 T.

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

With the advent of specialized endorectal sensors (radio frequency coils), dynamic contrast and spectroscopy, MRI quickly attracted the attention of many clinicians and researchers and gradually entered into a series of diagnostic examinations of patients with prostate cancer. The slow development of this area of radiation diagnosis in our country was due to the insufficient prevalence of radical methods of treatment of prostate cancer (including prostatectomy and radiation therapy), the low availability of modern tomographs and the lack of appropriate training programs for specialists in radiation diagnostics and urologists. In recent years, the situation began to change for the better, as public procurement of medical equipment grew and specialized centers for the diagnosis and treatment of prostate cancer appeared.

Indications for the procedure

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

• differentiation of stages T2 and T3 for determining indications for surgical or radiation treatment in patients with an average and high risk of extraprostatic spread of the tumor;
• assessment of regional lymph nodes and detection of metastases in the pelvic bone and lumbar spine (more accurate diagnosis in comparison with CT);
• degree of differentiation of the tumor according to Gleason 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 PCa on the background of ongoing treatment;
• the detection of local recurrences of prostate cancer or metastases in regional lymph nodes in the biochemical recurrence of cancer after radical prostatectomy;
• PSA level> 10 ng / ml.

When formulating the 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 level of PSA and the degree of differentiation of the tumor.

The diagnostic efficiency of magnetic resonance imaging, depending on the presence of locally advanced prostate cancer

	Low risk (PSA <10 ng / ml, Gleason 2-5)	The average squeak (PSA = 10-20 ng / ml, Gleason 5-7)	High risk (PSA> 20 ng / ml, Gleason 8 10)
Detection of a tumor	Low	High	High
Determination of local prevalence	High	High	High
Identification of lymphadenopathy	Average	Average	High

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

Patients with negative biopsies (more than two) in history, PSA level within the "gray scale" (4-10 ng / ml), absence of pathology in the TRUS and digital rectal examination, showed MR-planning of the biopsy, during which the sites , suspicious of the presence of a neoplastic process.

[1], [2], [3], [4], [5], [6]

Preparation

Patients with suspicion of prostate cancer MRI of the small pelvis can be performed both before the transrectal biopsy (in the presence of the result of the determination of serum PSA) and 3-4 weeks after it (after the disappearance of the sites of post-biopsy hemorrhages 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 an MRI scan of the prostate (endorectal and superficial) is to cleanse the rectum with a small enema. The study is carried out with a filled bladder, if possible - after suppression of peristalsis by intravenous injection of gljaghon or giosnip of butyl bromide.

[7]

Technique of the mRI of the prostate

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

The patient is placed inside the tomograph in the position lying on the back. The study begins with a rapid tomography (localizer) to monitor the location of the sensor and plan subsequent programs. Then, T2-weighted images in the sagittal plane are obtained to assess the overall anatomy of the pelvis. T1-weighted images in the axial plane are used to assess zones of lymphadenopathy, the detection of blood in the prostate and metastases in the pelvic bones. Aimed axial T2-weighted tomograms with a cutoff thickness of about 3 mm are the most informative evaluation of the prostate gland. Rapid tomography to obtain T1-weighted images and suppression of the signal from the adipose tissue is used to conduct dynamic contrast of the prostate gland and evaluation of the lymph nodes. The total duration of the study is about 25-30 minutes.

The protocol of endorectal magnetic resonance imaging for prostate cancer

Pulse sequence	Plane	Cutting thickness / spacing, mm	A task
T2-VI (spin echo)	SP	5/1	Evaluation of the general anatomy of the pelvic organs
T1-VI (spin echo)	AP (through a small pelvis)	5/1	Search for lymphadenopathy, assessment of pelvic bones
T2-VI (spin echo) aimed at the prostate gland	AP	3/0	Evaluation of the prostate and seminal vesicles
	Kp / cn	3/0	Evaluation of the prostate and seminal vesicles
T1-VI (gradient echo) with suppression of the signal from adipose tissue, intravenous contrast and multiphase scanning	AP	(1-3) / 0	Evaluation of the prostate and seminal vesicles

Notes. The joint is the sagittal plane; AP - axial plane; KP is the coronal plane; VI - weighted image.

Scan is performed without delaying breathing. When performing tomography in the axial plane, it is necessary to use the transverse direction of phase coding (from left to right) in the fields of reduction of artefacts from pulsations of vessels and movement of the anterior abdominal wall. Also, it is possible to use a presatura in the area of the anterior abdominal wall. The processing of the obtained images should include a program for correcting the intensity of the surface coil signal (BFR), which provides a uniform signal from the entire pelvic area, and not only from the prostate gland.

Of the MR contrast preparations, 0.5 M-contrast preparations (GD-DTPA) are usually used, calculated at 0.1 mmol, or 0.2 ml, per 1 kg of body weight of the patient (the volume of the contrast preparation usually does not exceed 15-20 ml per one study). When carrying out MP studies with dynamic multiphase contrasting, it is preferable to use 1.0 M preparations (gadobutrol), since with a smaller volume of administration (7.5-10 ml) compared to 0.5 M preparations, it is possible to achieve a more optimal bolus geometry , due to which the information content of the arterial phase of contrasting is increased.

Contraindications to the procedure

Contraindications to MRI are related to the effect of the magnetic field and radiofrequency (non-ionizing) radiation.

Absolute contraindications:

• artificial pacemaker;
• intracranial ferromagnetic haemostatic clips;
• intraorbital ferromagnetic foreign bodies;
• implants of the middle or inner ear;
• insulin pumps;
• neyrostimuljatory.

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

Erectorectal MRI is also contraindicated within 2-3 weeks after multifocal transrectal biopsy of the prostate gland, within 1-2 months after surgical interventions on the anorectal area and patients with severe hemorrhoids.

[8], [9], [10], [11]

Normal performance

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

Normal MRI anatomy of the prostate

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

Pseudocapsule of the prostate is visualized in the form of a thin hypo-intesive border, which on its front surface merges with the fibromuscular stroma. On T1-weighted images, the zonal anatomy of the prostate gland is not differentiated.

The size and volume of the prostate gland is estimated by the formula:

V (mm ³ or ml) = x • y • z • 0.1

The rectum-prostate angles should be free, not obliterated. The rectum-prostate fascia between the prostate gland and the rectum is usually clearly seen on axial tomograms. On the posterolateral surface of the prostate gland on both sides neurovascular bundles should be determined. The dorsal venous complex, usually hyperintensive on T2-weighted images due to slow blood flow, is revealed on its front surface. Seminal vesicles are visualized as fluid cavities (hyperintensive on T2-weighted images) with thin walls.

In a study with dynamic contrast, the contents of the vesicles do not accumulate the drug. The membrane of the urethra is visualized on sagittal or frontal T2-weighted tomograms.

Normal lymph nodes are best seen on T1-weighted images on the background of fatty tissue. As with MSCT, the size of the nodes is the main sign of metastatic lesion.

Normal bone tissue on T1- and T2-weighted images is hyperintensive due to the high fat content in the bone marrow. The presence of hypo-intensive foci (in the bones of the gas, spine, femur) most often indicates a metastatic osteoblastic lesion.

[12], [13], [14], [15], [16], [17], [18], [19], [20], [21]

Benign prostatic hyperplasia

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

[22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]

Prostatitis

The basis of the diagnosis of prostatitis is a clinical examination in combination with microbiological studies. If suspected complications develop (abscess formation), as well as patients with unclear etiology, pelvic pain is usually performed by ultrasound or MRI. Hypo-intensive foci in the peripheral zone of the prostate gland on T1-weighted images can correspond to both inflammatory changes and foci of neoplasia, MR-criteria of the foci of prostatitis - cone-shaped form of hypointense foci, clear contours, lack of mass effect.

Cysts of the prostate

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

Most often intraprostatic cysts come from the prostatic mantle or the vas deferens, extraprostatic - from the seminal vesicles, the remnant of the Mullerian duct.

[33], [34], [35], [36], [37]

Adenocarcinoma of the prostate

For adenocarcinoma of the prostate, low signal intensity is characteristic for T1-weighted images against the background of a signal of high intensity from the normal peripheral zone of the prostate gland.

The most important advantage of endorectal MRI is the ability to accurately localize neoplastic lesions, determine the nature and direction of tumor growth. In particular, MRI reveals foci of cancer in the anterior parts of the peripheral zone of the prostate gland that are difficult to access in transrectal biopsy. Incorrect shape, diffuse spread with mass effect, fuzzy and uneven contours - morphological signs of low intensity signal centers in the peripheral zone of the prostate gland, suggesting the malignant nature of the lesion.

With dynamic contrast, cancer sites rapidly accumulate a contrast agent in the arterial phase and rapidly remove it, which reflects the degree of neo-histogenesis and, accordingly, the degree of malignancy of the tumor.

Representatives of the North American School of Radiation Diagnostics advocate the use of MR-spectroscopy instead of dynamic contrast, which is preferred by representatives of the European school of radiation diagnosis, for the precise localization of cancer sites. 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 you to directly visualize the capsule of the prostate and determine the local prevalence of the tumor.

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

• asymmetry of neurovascular bundles;
• obliteration of the rectum-prostate angle;
• bulging of the gland contour;
• extracapsular tumor;
• wide contact of the tumor with the capsule;
• asymmetrically hypo-intensive signal from the contents of the seminal vesicle.

Comparative characteristics of MP criteria for the exprostatic spread of cancer

MR criterion	Accuracy,%	Sensitivity,%	Specificity,%
Asymmetries of neurovascular beams	70	38	95
Obliteration of the recto-prostatic angle	71	50	88
Capsule bulging	72	46	79
Extracapsular tumor	73	15	90
Overall Impression	71	63	72

The expressed extracapsular invasion according to MRI not only determines the inexpediency of surgical treatment - it is considered an unfavorable prognostic factor.

Ways of involvement of seminal vesicles in prostate cancer:

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

The main features of seminal vesicle invasion:

• the absence of a gypsum-intensive signal from the content on T2-weighted images;
• asymmetric enlargement, hemorrhage into the vesicle.

Gioointensive foci in seminal vesicles can be associated with postbiopsy hemorrhage, amyloidosis (about 30% of men over 75 years old), compression of prostate adenoma.

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

A study with intravenous contrasting allows more accurate determination of the tumor boundary.

Hormonal ablation in PCa leads to a decrease in the intensity of the MR signal, a decrease in the size of the gland, which makes the diagnosis somewhat difficult. However, there is no significant decrease in the accuracy of MR staging with hormonal ablation.

Recently, MRI has increasingly attracted the attention of specialists as a method of planning treatment interventions (in particular, radiotherapy and surgical interventions), since modern methods of treatment in many cases allow the patient to be cured of cancer, and the issue of quality of life after treatment. For this reason, radiotherapy in PCa is carried out after marking the field of exposure to radiation from CT or MRT data, which allows to secure adjacent unaffected organs (for example, the neck of the bladder).

Carrying out an MRI before radical prostatectomy allows us to evaluate the membrane section of the urethra, the length of which is inversely correlated with the severity of urination disorders after surgery. In addition, the severity of the dorsal complex, a potential source of massive hemorrhage at its intersection during the operation, is assessed.

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

[38], [39], [40], [41], [42], [43], [44], [45], [46], [47]

Diseases of the prostate with a low signal intensity

Low signal intensity is also characteristic for inflammatory changes, especially chronic prostatitis, fibrous-cicatricial changes, fibromuscular or stromal hyperplasia, the consequences of hormonal or radiotherapy MRI without dynamic contrasting does not allow reliably differentiating most of the listed changes and diseases.

Posebiopsy changes in the prostate gland. Characteristic is the unevenness of the capsule of the prostate, hemorrhage, changes in the MP signal of the parenchyma.

A full-scale MR-study becomes possible only after the disappearance of hemorrhages, which on average takes 4-6 weeks (sometimes 2-3 months).

[48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58]

Operational characteristics of MRI of the prostate

The average sensitivity of MRI in detecting PCa (primarily microscopic foci) does not allow this method to be used to exclude the neoplastic process.

With biochemical cancer recurrence after radical prostatectomy, MRI allows to identify with localized tumor recurrence or metastasis to regional lymph nodes with 97-100% accuracy.

The accuracy of MRI in detecting foci of neoplastic lesion of the prostate is 50-90%. The sensitivity of MRI in the localization of PCa is about 70-80%, while microscopic foci of cancer can not be detected with MRI. The hypertension on T2-weighted images of mucinous adenocarcinoma of the prostate makes it difficult to diagnose and leads to false negative results of MRI.

Clinical information (PSA level, previous treatment), knowledge of the anatomy of the prostate, use of the endorectal sensor, dynamic contrast and spectroscopy make it possible to approximate the detection accuracy of foci of cancer by MRI to 90-95% (the specificity is more pronounced).

The sensitivity of MRI to the ex-extrastatic extension is in the range of 43-87%, which is primarily due to the impossibility of visualizing microscopic germination of the prostatic capsule. The sensitivity of revealing an extension less than 1 mm deep with endorectal MRI is only 14%, whereas with tumor growth beyond the gland more than 1 mm, the index rises to 71%. In the low-risk group (PSA <10 ng / ml, Gleason score <5), the frequency of detection of tumor spread beyond the prostate gland is low, macroscopic extensively observed rarely, which significantly increases the frequency of false-negative results. The sensitivity of detection of invasion of seminal vesicles is 70-76%. The highest specificity (up to 95-98%) and the prognostic value of a positive MRI result are achieved when examining patients with moderate or high risk of intracapsular invasion (PSA> 10 ng / ml, Gleason score 7 or more).

Factors affecting the outcome

One of the main problems in identifying cancer foci and extracapsular tumor spread is the high variability in the interpretation of tomograms by different specialists. With the help of MRI, reliable results can be obtained only when analyzing tomograms by qualified specialists in radiation diagnostics with extensive experience in urogenital radiology. Supplementation of standard MRI with dynamic contrast enhancement allows to more standardize the study and increase the accuracy of detection of extracapsular invasion. The main task of the specialist in radiation diagnostics is to achieve high specificity of MR diagnostics (even at the expense of sensitivity) in order not to deprive the operable patients of the chance of radical treatment.

Limitations of magnetic resonance imaging of the prostate:

• low sensitivity with microscopic lesion;
• false-positive results due to the presence of blood biopsies in the peripheral zone;
• the transition of prostatic adenoma to the peripheral zone;
• the detection of cancer in the central zone of the prostate;
• pseudolocations in the base of the gland;
• high dependence of the accuracy of diagnosis on the experience of a physician-radiologist.

[59], [60], [61], [62], [63], [64], [65], [66], [67]

Complications after the procedure

In the overwhelming majority of cases, endorektalnoe MR-study patients are well tolerated. Complications are extremely rare (a small discharge of blood in the presence of a patient with defects in the rectal mucosa).

Adverse reactions with the use of MR contrast preparations occur extremely rarely (less than 1% of cases) and usually have a mild degree of severity (nausea, headache, burning at the injection site, paresthesia, dizziness, rash).

[68], [69], [70], [71], [72], [73], [74], [75], [76]

Prospects of magnetic resonance imaging of the prostate

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

A significant diagnostic limitation of both CT and MRI is low accuracy in the diagnosis of metastatic lesions of lymph nodes in the absence of their quantitative and qualitative increase. The main hopes of solving this problem are associated with the development of molecular diagnostics and the development of lymphotropic contrast agents (currently undergoing clinical trials of II-III phase). As radiation diagnostics develops, the use of MRT in clinical practice, tumorotropic and lymphotropic contrast agents may become the most informative a comprehensive method for diagnosis of prostate cancer, mandatory for patients at medium and high risk, before biopsy or treatment.