1. Assessment of the correlation between GAGs and T2 relaxation time using an MR imager
Melissa Haehn, Can Pi, Benjamin Sprague, Andrea Zelisko – UW-Madison BME 402
Advisor: Kristyn Masters, PhD; Client: Victor Haughton, MD – UW-Madison Hospital – Dept. Neuroradiology
Abstract
To determine a correlation between glycosaminoglycans (GAGs) and T2 relaxation time by using magnetic resonance imaging. Gelatin hydrogels, of varying gelatin content, were created to study the affect of GAGs on T2 relaxation time. Chondroitin sulfate was added in increasing amounts to the gelatin samples between 9% and 23% to determine the T2 of different levels of GAGs. The gels were tested in a GE Signa Excite 1.5 T MR scanner using a QID sequence developed at the University of Wisconsin. Analyzing the data revealed a negative correlation between the amounts of GAGs in the sample to T2 relaxation time.
Figure 5: GAG concentration vs T2 measurement – During the 2nd Scan of Test Round 2, 24 samples were scanned in the MR scanner and a strong relationship was revealed. This graph shows the inclusion of the 0.5g GAG data, which did not greatly alter the linear fit or R2 value.
Motivation
Our client, Dr. Victor Haughton, is a neuroradiologist at the UW-Hospital with research interests in dynamic spine MR imaging.
Figure 4: The 3-dimensional computer modeling of the phantom – The phantom was designed considering distance between the sample and the MR coil, amount of water loading, and maximal elimination of artifact.
Figure 3: Scan image of phantom containing Gd samples of same concentration
Figure 6: Change in T2 measurement due to change in chondroitin sulfate – The comparison of T2 values belonging to samples containing differing amounts of chondroitin sulfate (weight per weight percentage) showed the trend of increasing T2 with decreasing chondroitin sulfate percent.
Figure 1: The goals of this project involve disk research and assessment of the MR scanner’s accuracy.
Verification Experiments
Due to much variation in gel testing, it was important to verify the accuracy of the MR scanner and a set of uniform gadolinium doped water samples were created for testing in the MR.
Gadolinium doped water solutions of T2 = 38ms created
Scanned in MR using the head coil
Very little intra-phantom variation found, T2 values ranging from 32 to 44 ms
Discussion of Results
Conclusions from correlations:
Strong linear correlation between chondroitin sulfate concentration and T2 hints that GAGs are interacting with water in way to minimize spin.
Decrease in slope of GAG-T2 relationship with increase of gelatin weight/weight percent shows that gelatin also interacts with water in way to minimize spin.
Hypothesize that a decrease in gelatin percentage leaves more water suspended in matrix to be affected by the GAG concentration.
Possible causes of variability:
Absence of difference in Test Round 3 between 5% GAG and other GAG samples made at later time shows 5% gel did not change over time.
Similarity of two test rounds of phantom and Gadolinium samples shows that MR scanner should be consistent between two scan occurrences.
Similarity between consecutive scans show consistency possible between scans
Because of testing limitations, speculate difference in temperature of gels at time of scans is source of variability.
Conclusions about gel composition:
GAG-H2O relationship shown in samples is inverse of relationship seen in intervertebral discs in body.
Only chondroitin sulfate used in gels, intervertebral discs have several types of GAGs
Further proof evidence that samples not a physiologic model of intervertebral discs
Future Work
Despite the great deal of work on this project, there is further research which could be conducted to advance this area of disk and MR research.
Imaging of cadaver disks – researcher contact formed
Additional scans of gadolinium samples to assess effect of coils
Further scans of gelatin samples to generate more data, reliability
Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging (MR imaging) is an imaging modality that uses the spin of hydrogen nuclei for visualizing the inside of human bodies.
Hydrogen nuclei aligned by powerful magnet
-Figures 1 and 2 illustrate this alignment and nuclei spin
Radio frequency (RF) pulse is applied perpendicular to magnetic field
After RF turned off, hydrogen nuclei realign with magnetic field and emit energy
Time to realign is T1 value, exponential decay of signal is T2 value
Table 1: Results of gadolinium scans showing little intra-phantom variation within the MR scanner when using the head coil.
It was also deemed important to assess the variation in T2 measurement due to the use of different coils.
Set of gels scanned in MR using head and spine coils
Variation between the two occurred
Significant signal lost in samples furthers from the spine coil
Head coil led to equal signal distribution throughout the phantom
Figures 1 (left) and 2 (right): The alignment and spin, respectively, of hydrogen atoms in a magnetic field.
Hydrogel Preparation and Selection
From the testing, we chose to use gelatin gels with the lowest T2
Targeted T2 was between 50 and 150 ms and so we made gels between 9 and 23% (wt/wt %)
-Sodium Azide added as a preservative
GAGs added in increasing amounts
-0.2, 0.5 and 0.8 grams chondroitin sulfate
-used chondroitin sulfate A from bovine trachea cell
Gel Testing Results
Scans with plain gelatin and gelatin with GAGs were performed on two days and a MATLAB program was used to analyze the results
Magnitude of T2­ values increased between the two days
-Maybe due to conditions of scan
-Still maintained the same trend
Data shows that GAGs had a lowering effect on T2 (R2 values)
-See Figure 5
-At the higher gelatin weights, GAGs had less of an effect as indicated by slope
-0.5g gel data and inclusion explanation
Compared change in T2 with respective change in GAGs
-Change in T2 is greater when change in GAG is greater
-Positive correlation
-See Figure 6
Spinal Anatomy and Imaging
Disks act as cushions between vertebrae
Disks degenerate with age due to decreased water and blood flow
Health problems include back pain, pinched nerves, muscle spasms
MR’s high proton sensitivity proves superior image quality to X-ray
Phantom
A phantom was designed and constructed specifically for the needs of this project. Since the primary goal was to assess how GAGs affect T2 measurement, a phantom that eliminated the differences within the magnetic field was desired.
Varying distance from MR coil – 2.75 to cm
Symmetric design for easy scanning in a single plane
Volume of 2.5 L for adequate loading
Friction fit tubes to eliminate material artifact
Made of acrylic
Constructed at UW Mechanical Engineering Lab
Acknowledgements
UW-Madison Hospital - Dr. Victor Haughton, Dr. P. Anderson, Dr. R. Patel, Kelli Hellenbrand
UW-BME Dept. – Prof. K. Masters, Prof. W. Block
UW-Engr. Manufacturing Lab – Todd Kile
UW-Medical Physics Dept. – Prof. E. Madsen, M. Hobsen, J. Perry