Specifics of small-joint MRI
The primary challenge when scanning small joints (e.g., distal interphalangeal joints) is the generation of an extremely low volume of MR signal. To obtain a high-quality image, diagnosticians must resort to technical workarounds:
- Using saline bags (saline bag) or specialized local coils (bespoke small parts coils).
- Acquiring a continuous dataset (contiguous dataset) by reducing slice thickness and completely eliminating interslice gaps.
Another difficulty is the chemical-shift artefact (chemical-shift artefact), which significantly degrades image quality, particularly on high-field scanners. To eliminate it, the receiver bandwidth (receiver bandwidth) and matrix spatial resolution are increased. Unfortunately, this inevitably leads to a decrease in the signal-to-noise ratio (SNR). Moreover, if the joint axis is oriented perpendicularly to the main magnetic field $B_0$, standard spectral fat suppression may fail. In such cases, using the STIR (short tau inversion recovery) sequence is recommended for adequate bone marrow visualization.
Combating metal artefacts
Metallic structures (implants, screws) become magnetized and distort the homogeneity of the $B_0$ field. This leads to signal loss due to dephasing and misregistration artefacts such as geometric distortion and summation lines. Artefacts occur both in-plane and through-plane.
How to minimize distortions:
- Prosthesis characteristics: Titanium alloys produce fewer artefacts than steel ones. Long, thin structures oriented parallel to $B_0$ cause fewer distortions. The most severe artefacts are generated by spherical prostheses (hip joint) and massive weight-bearing components (knee joint).
- Parameter optimization: A sensible step is swapping the frequency- and phase-encoding directions, which shifts the artefact shadow away from the region of interest. Gradient-echo and long-echo $T_2$-weighted sequences should be avoided in favour of $T_1$ and PD fast-spin echo. Increasing bandwidth, reducing voxel size, and decreasing slice thickness mitigate susceptibility effects, albeit at the expense of SNR.
- Advanced technologies: Scanner manufacturers offer multispectral imaging techniques and view-angle tilting. However, these may cause blurring and have limitations regarding available weightings.
Cartilage mapping and MR elastography
T2 cartilage mapping (T2 cartilage map) makes it possible to assess zonal variations in collagen and water content. For instance, in the medial tibiofemoral compartment of the knee, cartilage degeneration is invariably accompanied by an increase in free water content. On mapping colour scales, maximum hydration is typically displayed in blue, and an increase in relaxation time is directly associated with early cartilage damage.
In addition to traditional evaluation of articular surfaces, MR elastography (MR elastography) is utilized. This technique allows visualization of differences in elasticity and stiffness between the relaxed and contracted muscle tissue surrounding the joint. It is also worth noting the use of ultrashort echo time (TE): at ultra-low values (around 0.03 ms), the osteochondral junction becomes hyperintense, whereas it appears dark at standard values.
Alternative methods: scintigraphy and arthrography
Although MRI remains the gold standard, other modalities are also employed in the diagnosis of joint and bone pathologies:
- Radionuclide bone scintigraphy. Sensitive to areas of high osteoblastic activity. A normal scintigram has a high negative predictive value. However, the method fails in multiple myeloma (where osteoblastic activity is reduced) and is limited by low spatial resolution.
- SPECT-CT (SPECT-CT). A hybrid technique indispensable for the precise localization of osteoid osteomas in the spine, pars interarticularis stress fractures, periprosthetic infections, and stress fractures (e.g., of the sacrum following spinal fusion).
- Traditional arthrography. Administration of iodinated contrast under X-ray guidance. It has been largely superseded by MRI, but remains relevant for shoulder hydrodilatation in adhesive capsulitis and for preoperative foot joint injections.
Briefly: answers to topical questions
In which pathologies does radionuclide bone scintigraphy yield false-negative results due to reduced osteoblastic activity?
Radionuclide bone scintigraphy may produce false-negative results in diseases and metastases that do not elicit a sufficient osteoblastic response. Such pathologies include:
- Multiple myeloma (multiple myeloma), a systemic disease characterized by reduced osteoblastic activity in which the method is ineffective.
- Breast carcinoma (breast carcinoma), as certain metastases are not detected on the scintigram due to the absence of an osteoblastic reaction, but are revealed by MRI.
What are the indications for conventional X-ray arthrography in modern practice?
In modern practice, conventional X-ray arthrography retains its relevance in two main clinical scenarios.
- Shoulder hydrodilatation (hydrodilatation) is used for diagnostic confirmation and treatment (controlled joint distension) in adhesive capsulitis.
- Pre-operative foot joint injections are used to visualize communications between the rearfoot and midfoot joints (hind- and mid-foot). This is critical for precisely determining the target injection zone when planning arthrodesis.
Which anatomical structures of the joint become hyperintense on MRI with ultrashort echo time (UTE)?
With ultrashort echo time MRI, the osteochondral junction of the joint becomes hyperintense. On standard MR images (e.g., with an echo time of 48 ms), this anatomical structure appears hypointense. However, when using an ultrashort echo time (0.03 ms), its signal intensity increases significantly, allowing detailed visualization of this zone.
Whole-body MRI is used for staging which oncological and systemic diseases?
Whole-body MRI is used to stage a number of oncological and systemic conditions due to its high sensitivity. These diseases include:
- Multiple myeloma (multiple myeloma), the first-line method for staging and detecting asymptomatic bone lesions.
- Chronic recurrent multifocal osteomyelitis (CRMO), the standard staging tool in children and adolescents.
- Bone lymphoma (lymphoma), used to visualize multifocal skeletal involvement.
- Bone metastases (bone metastases), used in the workup algorithm for confirmed primary malignancies.
The T-rule for metal artefacts: Titanium is better than steel, T1 is preferred, and long-echo T2 is Taboo.
Class and exam questions
How to combat signal deficiency in MRI of small finger joints?
Saline bags or specialized coils are used, and slice thickness is reduced to produce a contiguous, gapless data set.
Which scanner settings save the scan in the presence of a metal prosthesis?
Gradient-echo sequences should be avoided, T1 or PD FSE should be used, and STIR should be chosen for fat suppression. Swapping the frequency- and phase-encoding directions is also helpful.
What does the blue color on a cartilage T2 map mean?
The maximum water content is marked in blue, indicating an increased relaxation time and an early stage of cartilage degeneration.
In which cases is conventional arthrography still used?
It remains effective for shoulder joint hydrodilatation in the treatment of adhesive capsulitis and for pre-operative injections into the joints of the foot.
What else to study on this topic
- Mechanisms of chemical shift artifact formation
- Principles of multispectral imaging and view angle tilting
- Specific octreotide scintigraphy in oncogenic osteomalacia
- SPECT-CT in the diagnosis of stress fractures and prosthesis failure
- Pathophysiology of signal alteration from the osteochondral junction in ultrashort echo time
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