Ultra-High-Field Sequence Imaging for Appraisal of Cartilage Damage

Becky McCall

March 11, 2011

March 11, 2011 (Vienna, Austria) — A novel imaging method called glycosaminoglycan chemical exchange saturation transfer (CEST), which assesses cartilage damage using a 7-Tesla whole-body high-field magnetic resonance scanner, has been developed for use in patients undergoing cartilage repair surgeries for the first time.

Initial results from a study of this technique were presented here at the European Congress of Radiology (ECR) 2011.

Siegfried Trattnig, MD, head of the MR Center of Excellence in Vienna, Austria, who presented the findings, said a technique is now available for visualizing and quantifying the proteoglycan content of a joint, which is indicative of cartilage damage. The method will enable follow-up examinations of cartilage after surgery. Glycosaminoglycan CEST can also be transferred from a 7-Tesla to a 3-Tesla scanner, which would make the method more widely available because of the higher number of 3-Tesla scanners.

"This is important because everyone will suffer from osteoarthritis with age. It may also help with lower back pain, because [glycosaminoglycan] CEST can also be used for the spine. [Glycosaminoglycan] CEST now provides us the opportunity to detect the early stages of these diseases, and this might even allow us to reverse the process before it progresses to disc degeneration and arthritis," said Dr. Trattnig.

Sodium imaging can also assess cartilage vitality or damage by providing a contrast that is sensitive to glycosaminoglycan content, but sodium imaging requires 7-Tesla machines, which are in limited supply. "It was hard to develop CEST in a way that is clinically useful. But together with Siemens and Dr. Benjamin Schmitt from the University of Heidelberg . . ., we have managed to develop a CEST method that is ready for clinical application," Dr. Trattnig explained.

Twelve patients were scanned using a Siemens 7-Tesla whole-body system with a new 28-channel Tx/Rx knee coil. A 3-dimensional spoiled gradient echo sequence with centric reordering in a phase-encoding direction was used for CEST imaging. All the image sets were normalized against an image series acquired without saturation. CEST contrast was generated by subtracting an image recorded after selective saturation at a positive offset from the water resonance, taken from a reference image recorded at the same offset with opposite sign. Sodium imaging was performed with a circularly polarized sodium knee coil, using a modified GRE sequence.

The average CEST signal intensity using an offset of 1.7 ppm from water (the resonance of glycosaminoglycan −OH groups) in healthy volunteers was determined to be 6% ± 2%. In the cartilage transplant patients, significantly decreased signal intensities (−1.5% ± 2%), compared with control subjects, were found. In addition, signal intensities were significantly lower with normal-appearing cartilage.

The researchers compared CEST with sodium imaging, which is the standard method of assessment, to see whether they got similar results. A good correlation between CEST and 23Na imaging was found in all patients who had undergone cartilage surgery.

Dr. Trattnig explained that 2 years ago, the principle of CEST was proposed for the first time at the New York University Medical Center as a possible tool to visualize and quantify one component of articular cartilage — proteoglycans — which are also found in the intervertebral disc. However it was technically very demanding to use this method clinically. "Over several months of intense work with all the experts from our site, Benjamin Schmitt, and specialists from Siemens, we succeeded in the clinical application of CEST."

He explained that another proteoglycan-specific technique in use, dGEMRIC, has several disadvantages because it requires intravenous contrast administration and because the patient has to wait for 90 minutes after the administration of the contrast agent before scanning can be performed, limiting clinical use.

In conclusion, Dr. Trattnig said that cartilage transplantation is a good model because there is a focal area in cartilage that is different in healthy cartilage. "We found a very high correlation between the 2 methods. CEST visualized the proteoglycan content very well in the repair tissue."

Commenting on the work, Maximilian Reiser, MD, director of the Institute of Clinical Radiology at the Ludwig Maximilian University of Munich Medical Center, Germany, and president of the ECR, said that the CEST study by Dr. Trattnig and colleagues is exciting and on the verge of being translated into clinical practice. "It's a specific type of pulse sequence; with ultra-high-field, there are several advantages. Ultra-short TE provides the possibility of obtaining signals from tissues such as cartilage and tendons or structures with a very low proportion of protons."

Tendons, cartilage, and bones do not give a signal during normal imaging, Dr. Reiser explained.

"CEST sequencing with visualization of the proteoglycans may be a new step forward in the detection of early changes indicating the presence of osteoarthritis, which is becoming more and more important in the elderly, now that treatments are becoming available for early treatment of osteoarthritis," he added.

Dr. Reiser cautioned, however, that although the technology might be useful for several indications, there is still a lot of research required.

Siemens provided the 7-Tesla scanner. Dr. Trattnig and Dr. Reiser have disclosed no relevant financial relationships.

European Congress of Radiology (ECR) 2011: Abstract B-177. Presented March 4, 2011.

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