Vertebral anatomy and bone marrow physiology
The spine consists of cervical, thoracic, lumbar, sacral, and coccygeal segments. A vertebral body is a central framework of vertical trabeculae surrounded by a thin cortical layer. It acts as a calcium and phosphate depot and provides structural support for the anterior column of the spine.
The central part of the vertebral body is filled with red bone marrow, which is hematopoietically active. However, red and yellow bone marrow are not strictly separated and contain elements of each other. Bone marrow is a highly dynamic structure. With age, erythrocyte mass and trabecular bone volume decrease, while adipose tissue content increases. In the spinal column, these involutional changes occur later than in the bones of the appendicular skeleton.
Normal age-related patterns
T1-weighted imaging (T1WI) is optimal for bone marrow evaluation. Ricci et al. identify 4 normal patterns of bone marrow signal distribution depending on age:
- Pattern 1: Homogeneous low signal with a bright line around the basivertebral vein. Characteristic of young individuals (under 30 years).
- Pattern 2: Band-like high signal limited to the periphery of the vertebral body.
- Pattern 3: Multiple small, poorly defined high-intensity foci throughout the vertebra. (Patterns 2 and 3 are typical for individuals over 40 years).
- Pattern 4: An advanced form of the third pattern — multiple large (5–15 mm) high-signal foci on T1WI.
Multiple myeloma (MM)
In multiple myeloma, skeletal involvement occurs in 80% of cases (usually the axial skeleton and proximal ends of long bones). MRI is more sensitive than radiography in detecting infiltration, although skull vault and rib metastases are sometimes better visualized on standard radiographs.
There are 5 patterns of MRI findings in MM:
- Normal: observed with a low grade of malignant tumor infiltration.
- Focal (18–50%): foci greater than 5 mm, showing decreased signal on T1WI and increased signal on T2WI/STIR. Sometimes visible only on fat-suppressed T2WI.
- Diffuse (25–43%): generalized signal drop on T1WI (intervertebral discs appear brighter than vertebrae), hyperintensity on STIR/fat-suppressed T2WI. Accompanied by total gadolinium enhancement and indicates a high tumor burden.
- Combined: a combination of focal lesions and diffuse background changes.
- Variegated (1–5%): multiple small foci (dark on T1, bright on T2/STIR) against a background of normal bone marrow. Found in early stages.
Assessment of treatment response and complications
MRI is the method of choice for evaluating compression vertebral fractures, which occur in 55–70% of patients with MM. The method distinguishes benign collapse (due to diffuse osteopenia) from malignant collapse (due to tumor infiltration).
Evaluation of treatment efficacy:
- Good response: bone marrow signal intensity normalizes, FDG uptake on PET/CT decreases, apparent diffusion coefficient (ADC) on DWI increases.
- Partial response: diffuse pattern is replaced by a focal or variegated pattern.
- Non-viable remnants: foci persist but acquire fluid-like MRI signal, stop enhancing (or show rim enhancement), and may form a "fat halo".
Myelofibrosis, osteopenia, and transient osteoporosis
Proliferation of fibrous tissue and new trabecular bone formation in myelofibrosis lead to narrowing of the medullary cavity and osteosclerosis (more commonly in the axial skeleton). On MRI, this manifests as a total or heterogeneous hypointense (dark) signal on both sequences (T1WI and T2WI). Following successful allogeneic stem cell transplantation, the MRI appearance may regress to normal.
Other bone marrow changes:
- Disuse osteopenia: prolonged lack of weight-bearing (e.g., after an ankle fracture) can cause increased signal intensity on MRI (intermediate-weighted imaging with fat suppression) in adjacent joints (e.g., patella and femur).
- Transient osteoporosis: presents with a pattern of homogeneous bone marrow edema. Unlike fractures or avascular necrosis, transient osteoporosis lacks focal linear signal abnormalities and articular surface deformities.
The T1WI rule in myeloma: "Bright discs on a dark background." If intervertebral discs appear hyperintensity compared to vertebral bodies on T1, look for a diffuse pattern of tumor infiltration.
Class and exam questions
What are the characteristics of the MR signal in myelofibrosis?
Reactive fibrous tissue proliferation and diffuse osteosclerosis (new bone formation) occur. The bone marrow loses its cellular and fatty components, causing signal intensity to drop across all pulse sequences.
What is the value of whole-body MRI in multiple myeloma?
Whole-body MRI is more sensitive than CT and X-rays. Furthermore, it is included in the Durie and Salmon PLUS staging system and can significantly alter the disease stage based on the number of detected lesions (including diffuse ones that PET/CT might miss).
How do non-viable tumor lesions appear on MRI after treatment?
They may persist in the bone marrow but acquire fluid signal characteristics, do not enhance with gadolinium (or show only rim enhancement), and may be surrounded by a "fat halo".
What else to study on this topic
- Radiological signs of destruction: "punched-out" and "moth-eaten" lesions
- Durie and Salmon PLUS staging system for multiple myeloma
- Differential diagnosis of benign and malignant compression fractures
- Leukemic transformation in myelofibrosis and extramedullary masses
- Application of functional MRI (DWI) for assessing bone tumor cellularity
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