Assessment of the correlation between GAGs and T2 relaxation time using an MR imager

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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 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 3 Scan image of phantom containing Gd
samples of same concentration
Figure 1 The goals of this project involve disk
research and assessment of the MR scanners
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

• 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

• 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
• MRs 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 13.75 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