Chemical exchange in knee cartilage assessed by R1ρ (1/T1ρ) dispersion at 3T.

PURPOSE: To quantify the characteristics of proton chemical exchange in knee cartilage in vivo by R1ρ dispersion analysis. MATERIALS AND METHODS: Six healthy subjects (one female and five males, age range 24 to 71 y) underwent T1ρ imaging of knee cartilage on a 3T MRI scanner. Quantitative estimates of R1ρ (=1/T1ρ) were made using 5 different spin-lock durations for each of 12 different spin-lock amplitudes over the range 0 to 550Hz. When the variations of R1ρ with spin-locking strength (the R1ρ dispersion) are dominated by chemical exchange contributions, R1ρ dispersion curves can be analyzed to derive quantitative characteristics of the exchange and provide information on tissue composition. In this work, in vivo R1ρ dispersion of human knee articular cartilage at 3T was analyzed, and the exchange rates of protons between water and macromolecular hydroxyls (mainly in glycosaminoglycans) were estimated based on a theoretical model. RESULTS: R1ρ values showed marked dispersion in articular cartilage and varied by approximately 50% between low and high values of the locking field, a change much greater than in surrounding tissues, consistent with greater contributions from chemical exchange. From the theoretical model, the exchange rates in cartilage were estimated to be in the range of 1.0-3.0kHz, and varied within the tissue. Variations within a single knee appear to be larger with increasing age. CONCLUSION: R1ρ dispersion analysis may provide more specific information for studying cartilage biochemical composition and form the basis for quantitative evaluation of cartilage disorders.

A cadaveric simulation of distal femoral traction shows safety in magnetic resonance imaging.

OBJECTIVE: The purpose of this study was to evaluate the safety of a distal femoral traction pin subjected to a 1.5-T magnetic resonance image (MRI) with regard to pin migration and implant heating in a cadaveric model. METHODS: Deflection angles of various traction pins as well as a Bohler-style Steinmann Pin Tractor Bow (tractor bow) and a Kirschner wire bow subjected to a 1.5-T clinical MRI were measured. Tractions pins were placed into a cadaveric femur and the tractor bow was attached to the most distal pin to simulate distal femoral traction. Temperature and migration were measured after subjecting the cadaveric leg to a "worst-case scenario" MRI sequence for 30 minutes. RESULTS: All traction pins and bows showed deflection. The Kirschner wire bow showed a hazardous level of deflection and was immediately removed from further testing. The pin temperature changes were not significantly different than the changes in the MRI room temperature and a conduction loop was not seen in the combination pin and tractor bow. There was no significant migration of any pin nor was there objective loosening from pin vibration. CONCLUSIONS: Implant-quality stainless steel traction pins show no signs of adverse heating or pin migration when subjected to 1.5-T MRI clinical scanning. Kirschner bows are highly ferromagnetic and should not be used unless individually tested for safety. Steinmann Pin Tractor Bows that show weak ferromagnetism preliminarily appear safe to use during a 1.5-T MRI and do not produce a conduction loop with excessive heating in a cadaveric model, although further testing is indicated.

A simple, accurate method to confirm placement of intra-articular knee injection.

BACKGROUND: Intra-articular knee injections are routinely performed in clinical practice without documenting intra-articular placement. HYPOTHESIS: A small amount of air to an intra-articular knee injection produces an audible "squishing" sound with range of motion. STUDY DESIGN: Prospective nonrandomized clinical trial. METHODS: The study group (20 knees from 20 patients) received an intra-articular injection with a mixture of local anesthetic, corticosteroid, contrast dye, and 1 to 2 cc of air. The control group (10 knees from 5 patients) received extra-articular injections of a mixture of local anesthetic, contrast dye, and 2 cc of air. All knees were examined immediately after injection for a squishing sound with range of motion. Postinjection arthrographic radiographs were taken to verify the actual placement. RESULT: All study group knees and no control group knees had intra-articular contrast by radiograph. Clearly audible squishing sounds were heard in 17 of 20 study knees (sensitivity of 85%). Squishing sounds were audible in none of the control knees (specificity of 100%). CONCLUSION: Adding 1 to 2 cc of air to knee injections provides a no-cost, reliable, sensitive, and specific method of confirming accurate placement. CLINICAL RELEVANCE: This simple method is easily reproduced, can confirm accurate placement, and can eliminate extra-articular injection as the reason for clinical response failure.