Cartilage Mapping

How does cartilage mapping software work and what is it used for?  

​Cartilage mapping software is used to detect abnormalities of articular cartilage based on changes in relaxation times, especially T2. Tradenames include: Cartigam (GE), MapIT (Siemens), Cartilage Assessment (Philips), T2 RelaxMap (Hitachi), and Multi-echo T2 Mapping (Toshiba).

Relaxation times as well as biomechanics of articular cartilage reflect its intrinsic microstructure. About ⅔ of cartilage is composed of water while the other ⅓ consists of Type II collagen with negatively charged proteoglycans interspersed between its fibrils. The superficial and transitional zones have higher water content and more loosely organized collagen, resulting in longer T2 values. Deeper layers have lower water content and shorter T2s, with collagen organized into dense radial bands. Trauma, degeneration, and inflammatory changes in articular cartilage are generally manifest by increased free water content and loss of proteoglycans, causing increased T2 relaxation times. 

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A typical cartilage mapping sequence might consist of a set of 3-mm-thick slices obtained perpendicular to the cartilage surface. Four to six echoes at TE_i's from about 5 to 100 ms are obtained with TR and all other parameters held constant. These can be either spin echoes (to estimate T2) or gradient echoes to estimate T2*. T2⁽*⁾ calculations on a pixel by pixel basis can then made by fitting the recorded signal (S) to a first order model described by S = Ke^{−TE_i/T2(*)}. This is similar to methods used for quantifying iron deposition in the liver or heart described in a prior Q&A. After computation of relaxation times, color-coded maps are created to allow visual identification of T2⁽*⁾ changes within the cartilage.

T2 mapping of the knee articular cartilage. Note shorter T2 values (orange) of deeper layers near the bone. Courtesy of Philips Medical Systems.

The integrity of the proteoglycan component of cartilage has been investigated by postcontrast T1-mapping under dGEMRIC (delayed Gadolinium-Enhanced MRI of Cartilage) protocols as well as using T1ρ ("T1-rho") imaging. These are briefly described in the Advanced Discussion section below.

Advanced Discussion (show/hide)»

While T2-mapping techniques reflect primarily changes of water content and relaxivity, two other MR methods are focused on the structure and integrity of the proteoglycan component. Proteoglycan aggregates are largely responsible for cartilage elasticity and resilience. These macromolecules have a central protein core with covalently attached side chains of negatively charged glycosaminoglycans (GAGs), the most famous being chondroitin sulfates. Loss of proteoglycans is believed to be the initiating event in early osteoarthritis.

The dGEMRIC (delayed Gadolinium Enhanced MRI of Cartilage) method exploits changes in the negatively charged cartilage milieu to detect alterations in proteoglycan structure. The protocol begins with intravenous injection of a negatively-charged gadolinium chelate such as Gd(DTPA)²⁻. Such chelates are normally repulsed by the electronegative GAGs, but may accumulate in cartilage if the proteoglycan structure is disrupted.

After contrast injection, joint exercise is required, followed by a delay of at least 2 hours to allow penetration of the Gd(DTPA)²⁻ into cartilage. A T1-mapping sequence is then performed, which may be a Look-Locker method as commonly used in cardiac imaging or a spoiled GRE sequence obtained at two different flip angles. Shortening of T1 relaxation times indicates accumulation of contrast and damage to the proteoglycan structure.

Spin lattice relaxation in the rotating frame, more commonly known as T1ρ ("T1-rho") imaging has also been applied to study the integrity of articular cartilage. A typical T1ρ pulse sequence begins with a 90º-pulse along the x-direction that tips the initial magnetization into the transverse plane to point along the y-axis. Next, a low-powered B₁ (RF)-pulse, called a spin-locking pulse, is applied along the y-axis for 20-120 ms. The transverse magnetization precesses around this B₁ locking field in the rotating frame, and regrows via T1ρ relaxation while the pulse is being applied. A second 90º-pulse, this time aligned along the (−x)-direction tips the spin-locked magnetization back to the z-axis. A crusher gradient then dephases any residual transverse magnetization. Readout of the restored z-axis magnetization can then be accomplished by a turbo-spin echo sequence.

Because it represents a regrowth of magnetization along the applied B₁ field, T1ρ-relaxation has similarities to T1-relaxation (which regrows along the B₀ direction. Recall that T1-relaxation is sensitive to molecular motions near the Larmor frequency (which lie in the MHz range due to the large size of B₀). By contrast, T1ρ-relaxation is sensitive to a much slower range of motions (hundreds to thousands of Hz, determined by the much smaller B₁ field. In collagen-rich tissues like cartilage, chemical exchange and dipole-dipole interactions most affect T1ρ. Hence T1ρ measurements may reflect the state of collagen and proteoglycan structure which becomes disrupted by disease.

To calculate T1ρ the strength of the spin-locking pulse is kept constant while its duration is varied. The signal intensity, when plotted as a function of spin-lock time is a decaying exponential, from which T1ρ can be estimated.

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References
     Akella SV, Regatte RR, Gougoutas AJ, et al. Proteoglycan-induced changes in T1rho-relaxation of articular cartilage at 4T. Magn Reson Med 2001;46:419–423     
     Bashir A, Gray ML, Boutin RD, Burstein D. Glycosaminoglycan in articular cartilage: in vivo assessment with delayed Gd(DTPA)²--enhanced MR imaging. Radiology 1997; 205:551-558. (original description of the technique that would later be known as dGEMRIC).      
     Mosher TJ, Dardzinski BJ. Cartilage MRI T2 relaxation time mapping: overview and applications. Semin Musculoskelet Radiol 2004; 8:355-368.
    Ulmer JL, Mathews VP, Hamilton CA, Elster AD, Moran PR. Magnetization transfer or spin-lock? An investigation of off-resonance saturation pulse imaging with varying frequency offsets. AJNR Am J Neuroradiol 1996; 17:805-819. (See Appendix for a simplified discussion of rotating frame and spin-locking.)

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Related Questions
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