Functional imaging of the musculoskeletal system

Abstract

Functional imaging, which provides information of how tissues function rather than structural information, is well established in neuro- and cardiac imaging. Many musculoskeletal structures, such as ligaments, fascia and mineralized bone, have by definition a mainly structural role and clearly don't have the same functional capacity as the brain, heart, liver or kidney. The main functionally responsive musculoskeletal tissues are the bone marrow, muscle and nerve and, as such, magnetic resonance (MR) functional imaging has primarily addressed these areas. Proton or phosphorus spectroscopy, other fat quantification techniques, perfusion imaging, BOLD imaging, diffusion and diffusion tensor imaging (DTI) are the main functional techniques applied. The application of these techniques in the musculoskeletal system has mainly been research orientated where they have already greatly enhanced our understanding of marrow physiology, muscle physiology and neural function. Going forwards, they will have a greater clinical impact helping to bridge the disconnect often seen between structural appearances and clinical symptoms, allowing a greater understanding of disease processes and earlier recognition of disease, improving prognostic prediction and optimizing the monitoring of treatment effect.

Keywords: BOLD; Functional; diffusion; diffusion tensor imaging (DTI); imaging; magnetic resonance (MR); perfusion; spectroscopy.

Figures

Figure 1

High resolution proton density image of the wrist. Note the clear visibility of the intermediate intensity hyaline articular cartilage (arrow) and the hypointense fibrocartilage of the articular disc (open arrow).

Figure 2

Time-intensity curve of dynamic contrast enhanced MRI following curve fitting of data points. The line representing Eslope and the Emax point are shown. MRI, magnetic resonance imaging.

Figure 3

(A-C) Time intensity perfusion curves of three different patients with fractures of the waist of scaphoid. The proximal pole of the right hand image (A) has good perfusion while of the left hand image (C) has no perfusion i.e., is avascular. The proximal pole of the middle image (B) is poorly perfused (i.e., is ischaemic) as the Emax and Eslope are less the 50% those present in the distal pole.

Figure 4

(A,B) Axial MRI images of the wrist showing enhancement maps of synovium pre- and post-treatment (Rx). The enhancement map represents all of those pixels which enhance to a greater degree than muscle tissue (using thenar or hypothenar muscle as a reference standard) and is indicative of enhancing synovium (*). The volume of enhancing synovium decreases appreciably following treatment. MRI, magnetic resonance imaging.

Figure 5

(A-D) Series of images showing set up and results for perfusion imaging of intervertebral disc in a rat. (A) Wooden cradle for holding the anaesthetized rat; (B) cannulation of rat tail vein; (C) regions of interest placed on the intervertebral discs. The normal rat has six lumbar vertebrae.

Figure 6

Proton MRS of lumbar vertebral body. MRS assesses the fat peak: water peak ratio. In this example, the marrow is predominantly comprised of red or functioning marrow with a relatively high water peak than fat peak. MRS, magnetic resonance spectroscopy.

Figure 7

Dixon fat quantification technique provides a measure of relative amount of water or fat on a pixel by pixel basis. When compared to T1-weighted imaging, where fatty marrow is hyperintense and red marrow is isotense to muscle, one can appreciate the predominantly fatty marrow of the proximal femur (arrow) compared to the predominantly red marrow of the acetabulum (open arrow).