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Keywords:

  • cartilage;
  • MRI;
  • gadolinium;
  • osteoarthritis;
  • monitoring

Abstract

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

The negatively charged contrast agent Gd-DTPA2– distributes inversely to the cartilage fixed charged density. This enables structural cartilage examinations by contrast-enhanced MRI. In line with the development of a clinically applicable protocol for such examinations, this study describes the temporal pattern of Gd-DTPA2– distribution in femoral knee cartilage at three different doses in healthy volunteers. Nineteen volunteers (ages 21–28 years) were examined with a 1.5T MRI system. Quantitative relaxation rate measurements were made in weight-bearing central parts of femoral cartilage using sets of five turbo inversion recovery images with different inversion times. The cartilage was analyzed before and four times (1–4 h) after an intravenous injection of Gd-DTPA2– at single, double, and triple doses: 0.1, 0.2, and 0.3 mmol/kg body weight, respectively. The increase in R1 postcontrast was linearly dose-related at all times. The highest R1 values were registered at 2 and 3 h postcontrast, suggesting 2 h to be optimal in the clinical situation. The triple dose indicated a subtle compartmental difference in men, with higher contrast distribution medially than laterally. Results suggest that the triple dose is needed to detect minor cartilage matrix differences. Magn Reson Med 46:1067–1071, 2001. © 2001 Wiley-Liss, Inc.

The articular cartilage is a highly specialized tissue that provides joints with low friction and load distribution. Type II collagen (50–60% of dry weight) and proteoglycans (aggrecan, 15–30% of dry weight) are the most abundant cartilage matrix molecules (1). The aggrecans, which consist of a core protein to which highly negatively charged chains of glycosaminoglycans (GAG) are attached, are linked to hyaluronan to form a high molecular weight complex (1). By attracting water, the aggrecans create a swelling pressure that is counteracted by a framework of collagen fibers. The characteristic viscoelastic property of cartilage is dependent on the integrity of these and other matrix molecules.

In cartilage disease, such as osteoarthritis (OA), there are metabolic alterations and loss of matrix molecules (2–4). The limited repair potential of diseased joint cartilage has stimulated efforts to develop new medical and surgical treatments aimed at restoring abnormal cartilage (5–7). It is conceivable that such treatments are more successful if initiated in the earlier stages of disease. Consequently, there is a need for noninvasive methods that can detect early molecular and structural cartilage changes leading to joint destruction, as well as methods to monitor treatment outcome. Radiography, the present gold standard for OA diagnosis, is insensitive for the detection of such early cartilage changes (8, 9).

MRI has improved our ability to image articular cartilage considerably (10, 11). However, conventional MRI has shown limitations in the assessment of cartilage (10). Indirect MRI arthrography is a technique that studies joint pathology after an intravenous injection of a contrast medium, such as gadolinium diethylene triamine pentaacetic acid (Gd-DTPA2–) (12). In such studies, the contrast medium outlines the border between the tissues investigated and the synovial fluid. To enable structural joint cartilage examinations at the molecular level, Bashir et al. (13) waited for Gd-DTPA2– to penetrate the cartilage before they analyzed T1 relaxation, based on the hypothesis that a negatively charged contrast agent will distribute in the cartilage in an inverse relationship to the negatively charged GAG concentration. They showed an approximately linear inverse relationship between GAG concentration and Gd-DTPA2– distribution in the cartilage assessed by T1-weighted (T1W) MRI in in vitro studies of bovine articular cartilage. After an intravenous injection of Gd-DTPA2–, in vivo T1W MRI of human knee cartilage correlated well with ex vivo T1W MRI of the same cartilage equilibrated in Gd-DTPA2– (14). Mlynárik et al. (15) compared T2, T1ρ, and Gd-DTPA2–-enhanced T1 relaxation times of macroscopically damaged and undamaged cartilage in patients who received Gd-DTPA2– intravenously prior to hip or knee replacement. The authors concluded that Gd-DTPA2–-enhanced MRI is the most promising technique to analyze GAG depletion in mechanically undamaged cartilage.

Recently, Burstein et al. (16) provided preliminary data on protocol issues for delayed Gd-DTPA2–-enhanced MRI of articular cartilage (dGEMRIC). To further develop a clinically applicable protocol for structural examinations of femoral knee cartilage, the in vivo dose-response distribution in knee cartilage of healthy subjects, and the time window for MRI after contrast injection, are needed. To address these issues we describe the temporal pattern of Gd-DTPA2– distribution in medial and lateral femoral cartilage at three different doses in healthy volunteers.

METHODS

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

Subjects

Nineteen healthy volunteers (nine males and 10 females), ages 21–28 years (mean: 24) were included. Exclusion criteria were: 1) history of knee injury or knee pain in either knee; 2) regular medication, excepting oral contraceptives and vitamins; 3) contraindication for MRI (i.e., metal prosthesis, claustrophobia, or serious allergy); 4) receiving or scheduled to receive another contrast medium within 1 week prior to, or 2 weeks after proposed examinations; and 5) abnormality at physical examination of the knee. Gender, age, and body mass index (BMI) were registered and related to the results. Before inclusion, the nature of the procedure was fully explained to all subjects and written informed consent was obtained. The study was approved by the ethics review committee.

MRI

The contrast agent used, Gd-DTPA2– ( Magnevist®, Schering Ag, Berlin, Germany), is a highly stable and biochemical inert chelate complex with a molecular weight of 548 dalton. Gd-DTPA2– is eliminated through glomerular filtration with a plasma halflife of approximately 90 min (17).

MR examinations were performed with a 1.5T MRI system (Magnetom Vision; Siemens Medical Systems, Erlangen, Germany). All subjects were examined twice, at 1-week intervals. On both occasions an identical MRI protocol was used, precontrast as well as postcontrast. Central parts of the weight-bearing lateral and medial femoral cartilage were identified using a routine sagittal series (4 mm, FOV = 120 × 120 mm2, matrix = 512 × 320). In the selected sagittal slices (5 mm), quantitative T1 measurements were made in a region of interest (ROI) using sets of five turbo inversion recovery images with different inversion times; TR = 2000 ms, TE = 15 ms, turbofactor = 11, FOV = 120 × 120 mm2, matrix = 256 × 256, TI = 100, 200, 400, 800 and 1600 ms. The ROI included the cartilage from the surface to the subchondral bone, and was positioned between the center of the tibial plateau and the rear insertion of the meniscus (ROI size = 150–250 pixels) (Fig. 1). To standardize the procedure, all ROIs were drawn by a single investigator.

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Figure 1. An illustration of an ROI in the weight-bearing lateral femoral cartilage.

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At the first examination, subjects received Gd-DTPA2– at a dose of 0.3 mmol/kg body weight (triple dose). At the second examination, 1 week later, nine subjects (four males) received a dose of 0.2 mmol/kg body weight (double dose) and the other 10 a dose of 0.1 mmol/kg body weight (single dose). The Gd-DTPA2– injection was given in an antecubital vein. The end of drug injection was set to time-point zero. To optimize the distribution of Gd-DTPA2– into the cartilage the subjects walked up and down two stories (42 steps), five times at an easy tempo, corresponding to approximately 7 min of exercise. Postcontrast MRI was performed at approximately 1, 2, 3, and 4 h, respectively.

The recorded ROI values were transferred to a computer for relaxation time (T1) and relaxation rate (R1 = 1/T1) calculations using a three-parameter fit (18).

Analysis of variance (ANOVA) and a t-test were used for the statistical analyses. To evaluate linearity between different doses, a regression analysis was made.

RESULTS

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

Precontrast R1 did not differ between the medial and lateral compartments at the first and second examinations. Furthermore, there was no difference in R1 between the first and second examinations, showing that no residual contrast medium was present at the second visit, 1 week later. The average R1 (1/s) in the 76 precontrast examinations was 1.05 ± 0.09.

After injection of Gd-DTPA2–, the R1 values increased as a result of the contrast medium distribution in the cartilage (Fig. 2). At the 1-h postcontrast imaging, mean R1 values were increased at all doses (P < 0.001). Between 1 and 2 h postcontrast there was a further increase in mean R1 values at the double and triple doses (P < 0.001), but not at the single dose. No increase occurred between 2 and 3 h at any dose. At the 4-h postcontrast imaging, the mean R1 values decreased at the double and triple doses (P < 0.004 and P < 0.001, respectively), but not at the single dose (Fig. 2).

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Figure 2. Mean R1 values (1/s ± SD) of medial and lateral femoral cartilage before and four times (minutes ± SD) after an intravenous injection of Gd-DTPA2– at single, double, and triple doses: 0.1 (N = 9), 0.2 (N = 10), and 0.3 (N = 19) mmol/kg body weight, respectively.

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There was a linear relationship between the injected dose of Gd-DTPA2– and the increase in R1 at all postcontrast imaging. This is illustrated in Fig. 3, which shows the three different doses in relationship to the individual mean R1 values of medial and lateral compartments at 3 h postcontrast (r2 = 0.93). The r2 values at 1, 2, and 4 h postcontrast were 0.77, 0.90, and 0.89, respectively.

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Figure 3. Individual R1 values (1/s) of medial and lateral femoral cartilage (mean) 3 h after an intravenous injection of Gd-DTPA2– at single, double, and triple doses: 0.1 (N = 9), 0.2 (N = 10), and 0.3 (N = 19) mmol/kg body weight, respectively. The curve represents the regression line (r2 = 0.93).

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Figure 4 shows individual R1 values in the medial and lateral compartments at 2 h after the triple dose. The figure indicates a tendency towards a higher R1 in the medial compared with the lateral femoral cartilage. The R1 values (1/s) were 2.35 ± 0.15 laterally, and 2.48 ± 0.12 medially in men (P = 0.056, t-test, N = 9). Corresponding R1 (1/s) values for women were 2.31 ± 0.18 laterally and 2.40 ± 0.18 medially (P = 0.29, t-test, N = 10). An ANOVA including all examinations at the triple dose showed a higher R1 medially than laterally in men (P < 0.05, N = 36), but not in women. No compartmental difference was found at the single and double doses. There was no difference in R1 between genders at either dose. No relationships were found between R1 and BMI (range 17–28 kg/m2) or age (range 21–28 years) in this rather homogenous group.

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Figure 4. Individual R1 (1/s) values in the medial and lateral compartments 2 h after an intravenous injection of Gd-DTPA2– at triple dose, 0.3 mmol/kg body weight (N = 19). (○) denotes female values and (•) male values. The elongated, thicker lines show the mean R1 values for women and men, respectively.

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DISCUSSION

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

Previously, a single- and a double-dose Gd-DTPA2–-injection were given to two volunteers (16). In that study of patellar cartilage, the average T1 value was found to be lower after the double dose than after the single dose. In this first dose-response study of Gd-DTPA2–-enhanced MRI in healthy femoral knee cartilage, we show a linear dose-related distribution 1–4 h postcontrast using three different doses. Our in vivo data confirm the experimentally calculated contrast distribution by Bashir et al. (19). The linearity indicates that no active transport is involved in the distribution of Gd-DTPA2–.

In deciding the optimal dose for dGEMRIC, a simplified model for the distribution of Gd-DTPA2– developed by Bashir et al. (13, 19) can be used. Using this model, and the in vivo T1 values obtained in the present study, it is possible to make predictions about expected T1 values in diseased cartilage. It is assumed that normal healthy cartilage is relatively homogenous and has a GAG concentration of approximately 60 mg/ml (19, 20). The relative changes in T1 for cartilages with GAG concentrations ranging from 60 to 0 mg/ml would be 24%, 35%, and 40% for single, double, and triple doses, respectively. The above estimations are performed assuming the relaxivity of Gd-DTPA2– to be 3.5 (mMs–1) in both solution and cartilage (19). It has been noted that the relaxivity for Gd-DTPA2– may vary for different macromolecular cartilage content at field strengths used on clinical MR scanners (21). However, this effect was recently found to be of minor concern, and should not significantly affect the comparison between different doses of Gd-DTPA2– in this study (22). Accordingly, this favors the triple dose (0.3 mmol/kg body weight) for dGEMRIC. Notably, the subtle compartmental difference in men in the present study was only detected with that dose. However, the double dose may be sufficient to assess matrix differences in diseased (GAG-depleted) cartilage (14). The single dose, with R1 values close to those of cartilage without contrast, is probably too low for dGEMRIC. Due to a mild trombophlebitis in some subjects after the triple dose, we suggest that the dose be given slowly (over 3–5 min) with subjects in a horizontal position, and that the catheter be flushed thoroughly with 0.9% saline after injection.

We chose to study weight-bearing medial and lateral femoral cartilage for clinical and technical reasons. In knee OA, cartilage matrix changes usually start in the medial compartment (8, 9). Thus, the lateral compartment may be useful as an intraindividual reference in patient studies. The tibial cartilage likely is equally interesting, but is thinner and not as easily outlined for ROI analysis as the femoral cartilage.

It is conceivable that there is a relationship between the cartilage thickness and the time at which the cartilage is saturated with the contrast medium. In a study of three volunteers, Trattnig et al. (23) examined the signal intensity after contrast injection in different anatomic parts of the knee. They found that the time for maximum enhancement ranged from 45 min in the ventral femoral condyle to 4.5 h in patellar cartilage. Another study shows that penetration of Gd-DTPA2– into 1-mm-thick femoral cartilage had occurred within 45 min after contrast administration, and that penetration of patella cartilage was achieved after 2.5 h (19). In the present study of weight-bearing femoral cartilage, T1 relaxation times increased between 1 and 2 h postcontrast. In the clinical situation, it is beneficial to examine a patient as soon as possible after injection. Our results suggest approximately 2 h postcontrast as the optimal time for analysis of femoral cartilage.

The T1 relaxation time of cartilage without contrast in our study was approximately 1 s, which is in accordance with previously reported results (11). With respect to T1 postcontrast, Burstein et al. (16) examined the medial and lateral femoral cartilage in two healthy volunteers. Two hours after an intravenous Gd-(DTPA)2– injection with the double dose (0.2 mmol/kg body weight) the T1 values were between 440–570 ms. These results are similar to the values from the medial and lateral compartments in the present study. Two hours after the double dose we registered T1 values of 480–560 ms (1 SD) (Fig. 2). Repeated T1 measurements in five knees, 2 weeks to 2 months apart, have shown a good reproducibility with values within 10–15% (16).

Mature articular cartilage is avascular, and nutrition mainly occurs via the synovial fluid; however, diffusion directly from the synovium into the cartilage has also been suggested (24). Joint movements enlarge the cartilage area that comes into contact with synovial tissue and fluid (25). Accordingly, joint exercise has shown to increase the concentration of the contrast agent in the synovial fluid and cartilage (12, 14, 25). In the present study, all subjects walked up and down stairs at an easy tempo for approximately 7 min. According to a study that examined the fluid flux of molecules in and out of cartilage explants in experimental walking cycles, the influx of small molecules, such as Gd-DTPA2–, likely is not increased by the loading of the cartilage per se (26). The influence of different exercise regimes with regard to Gd-DTPA2– distribution in the cartilage has not been studied.

The significance of the subtle R1 difference between compartments in men is unknown but may reflect structural cartilage properties, such as GAG content. In this respect it is intriguing that knee OA shows compartmental differences. Others have found, by an indentation technique applied at arthroscopy, that the lateral femoral cartilage is 12% stiffer than the medial femoral cartilage (P < 0.01) (27). In an experimental study to validate their method, the authors showed that the indentation measurements reliably identified structural matrix changes induced by enzymatic degradation (28).

It can be speculated that body habitus and loading pattern can generate compartmental cartilage matrix differences. Biomechanically the knee joint is very complex, and in vivo cartilage loading studies to confirm that speculation are not easily performed. However, it is possible that human joint cartilage may adapt to joint loading similarly to that in canine cartilage (29). As discussed above, it is not likely that load-bearing per se during the walking exercise used in this study increased the cartilage influx of Gd-DTPA2– (26). It should be emphasized that the signal differences between compartments in the present study were subtle, on the order of 5% 2 h after the triple dose. In comparison, contrast-enhanced MRI of OA cartilage and enzymatically GAG-depleted cartilage have shown an almost 50% decrease in T1 relaxation time compared to intact cartilage (13, 14).

In conclusion, this study shows that dGEMRIC is a clinically feasible method with a potential to monitor cartilage structure in vivo. Two hours postcontrast is probably the optimal time to examine femoral weight-bearing cartilage in the clinical situation. Results suggest that the triple dose is more sensitive than the double dose in detecting structural matrix differences. The diagnostic utility of the technique has to be confirmed by studies of patients with diseased cartilage.

Acknowledgements

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

We thank Jan Åke Nilsson for statistical analyses, and Edwin Heisterkamp and Clark Ohlsson for their assistance with MR scanning.

REFERENCES

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES
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