Anatomical sequences: T1- and T2-WI
Anatomical assessment of the gland is based on standard scanning sequences. The protocol requires multiplanar projections with a 3 mm slice thickness and no interslice gap. The coronal plane is set strictly perpendicular to the border between the prostate and the rectum.
- T1-weighted images (T1-WI): Healthy glandular tissue demonstrates a homogeneous isointense signal, making it impossible to evaluate zonal architecture. The main purpose of this sequence is the detection of hemorrhages (haemorrhage), which appear as hyperintense areas.
- T2-weighted images (T2-WI): Allow for a detailed study of the prostate zones. In elderly patients, the transition zone (TZ) is frequently affected by benign prostatic hyperplasia (BPH / BPH), forming heterogeneous nodules—a pattern known as "organized chaos" (organised chaos). Prostate cancer (PCa / PCa) is more difficult to identify against this background. Its typical signs include a lenticular shape, indistinct margins, and a homogeneous hypointense signal, described as the "erased charcoal drawing" sign (erased charcoal drawing sign).
Transition zone assessment using PI-RADS
To standardize the description of suspicious lesions in the transition zone, the PI-RADS scale is used, based on T2-WI findings:
- PI-RADS 3: The lesion does not meet the criteria for other categories, or it demonstrates a heterogeneous signal with obscured contours.
- PI-RADS 4: A homogeneous hypointense lesion with indistinct margins or a lenticular shape. The maximum dimension is < 1.5 cm.
- PI-RADS 5: The lesion has the characteristics of category 4, but its size exceeds 1.5 cm, or there are clear signs of extracapsular extension.
Functional imaging: DWI and ADC
Diffusion-weighted imaging (DWI-MRI / DWI) reflects the random Brownian motion of water molecules within tissues.
- Free diffusion: Observed in fluid-rich structures. In men over 50, the peripheral zone contains many such structures due to cystic degeneration, yielding high values on ADC maps (ADC map).
- Restricted diffusion: Occurs in tissues with high cellular density (e.g., tumors). Malignant foci show a hyperintense signal on high b-value images and a hypointense (dark) signal on ADC maps.
The lower the tumor differentiation (high primary Gleason score of 4 or 5), the more restricted the diffusion and the lower the ADC values. Generally, prostate ADC values increase with patient age.
Diagnosis of metastatic involvement
Lymph nodes: Standard MRI has low sensitivity (17–70%) because ADC values of normal and affected nodes overlap significantly. MR lymphography with ultrasmall superparamagnetic iron oxide (USPIO, e.g., ferumoxtran-10) nanoparticle-based contrast increases sensitivity up to 90.5%. Healthy nodes uptake iron via macrophages and lose signal, whereas metastases remain bright.
Bones: Extranodal metastases are most commonly osteoblastic and affect the axial skeleton. Standard 99mTc-MDP bone scintigraphy has low sensitivity and may miss early lesions. Whole-body MRI is more effective in differentiating metastases from fatty bone marrow (using T1-WI, STIR, and Dixon chemical shift imaging) and surpasses scintigraphy in sensitivity (97% vs 79%).
MRI-guided targeted biopsy
mpMRI allows for the identification of cancer-susceptible regions (cancer suspicious region) for targeted biopsy. This reduces the number of unnecessary punctures and the detection of clinically insignificant tumors. There are three navigation approaches:
- In-bore MRI biopsy: Performed directly inside the scanner using a transrectal guide for precise needle placement verification.
- Cognitive biopsy (Visual targeting): The physician performs the biopsy under ultrasound guidance, mentally mapping the MRI data onto the screen. This method is inexpensive but difficult when lesions are located in the anterior and apical parts of the gland.
- Fusion biopsy (Fusion software): Real-time software-based coregistration of MRI and TRUS data. Accuracy is high, but it can be reduced by prostate deformation from the US probe or patient movement.
To memorize the MRI appearance of transition zone cancer, use the charcoal association: a malignant lesion appears as an "erased charcoal drawing" spot (dark, homogeneous, with indistinct margins) against a mottled background of BPH "organized chaos".
Class and exam questions
Why is zonal prostate anatomy not evaluated on T1-WI?
On T1-WI, normal prostate tissue demonstrates a homogeneous isointense signal, and zonal boundaries are not visible. This sequence is used to detect hyperintense areas indicative of hemorrhage.
What is the essence of restricted diffusion in tumors?
In tumor tissue, cells are densely packed. Due to high cellularity, water molecules cannot move freely, resulting in a signal drop on ADC maps (restricted diffusion).
Why is ferumoxtran-10 effective for detecting involved lymph nodes?
This is an iron nanoparticle-based agent that is phagocytosed by macrophages in healthy lymphoid tissue, causing a signal drop on MRI. Tumor foci lack macrophages, do not accumulate iron, and retain a bright signal.
What else to study on this topic
- Specifics of dynamic contrast-enhanced (DCE) imaging.
- 3D axial scanning protocol to reduce partial volume averaging artifacts.
- Specifics of SPECT/CT in evaluating indeterminate bone lesions.
- Mechanism of Dixon chemical shift imaging in the detection of bone metastases.
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Other topics in the section "Diagnostic Radiology of the Abdominal Organs"
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