Influence of water and fat heterogeneity on fat-referenced MR thermometry.
OBJECTIVE
To investigate the effect of the aqueous and fatty tissue magnetic susceptibility distribution on absolute and relative temperature measurements as obtained directly from the water/fat (w/f) frequency difference.


METHODS
Absolute thermometry was investigated using spherical phantoms filled with pork and margarine, which were scanned in three orthogonal orientations. To evaluate relative fat referencing, multigradient echo scans were acquired before and after heating pork tissue via high-intensity focused ultrasound (HIFU). Simulations were performed to estimate the errors that can be expected in human breast tissue.


RESULTS
The sphere experiment showed susceptibility-related errors of 8.4 °C and 0.2 °C for pork and margarine, respectively. For relative fat referencing measurements, fat showed pronounced phase changes of opposite polarity to aqueous tissue. The apparent mean temperature for a numerical breast model assumed to be 37 °C was 47.2 ± 21.6 °C. Simulations of relative fat referencing for a HIFU sonication (ΔT = 29.7 °C) yielded a maximum temperature error of 6.6 °C compared with 2.5 °C without fat referencing.


CONCLUSIONS
Variations in the observed frequency difference between water and fat are largely due to variations in the w/f spatial distribution. This effect may lead to considerable errors in absolute MR thermometry. Additionally, fat referencing may exacerbate rather than correct for proton resonance frequency shift-temperature measurement errors.
Image Sciences Institute, University Medical Center Utrecht, Utrecht, Netherlands.
Paul
P
Baron
Image Sciences Institute, University Medical Center Utrecht, Utrecht, Netherlands.
Roel
R
Deckers
Image Sciences Institute, University Medical Center Utrecht, Utrecht, Netherlands.
Job G
JG
Bouwman
Image Sciences Institute, University Medical Center Utrecht, Utrecht, Netherlands.
Chris J G
CJ
Bakker
Image Sciences Institute, University Medical Center Utrecht, Utrecht, Netherlands.
Martijn
M
de Greef
Image Sciences Institute, University Medical Center Utrecht, Utrecht, Netherlands.
Max A
MA
Viergever
Image Sciences Institute, University Medical Center Utrecht, Utrecht, Netherlands.
Chrit T W
CT
Moonen
Image Sciences Institute, University Medical Center Utrecht, Utrecht, Netherlands.
Lambertus W
LW
Bartels
D001940:Breast / Q000000981:diagnostic imaging; D003198:Computer Simulation; D005223:Fats / Q000737:chemistry*; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008954:Models, Biological; D019047:Phantoms, Imaging; D013817:Thermography / Q000379:methods*; D014867:Water / Q000737:chemistry*
D005223:Fats; D014867:Water
MR thermometry; MR-HIFU; absolute thermometry; fat referenced thermometry; magnetic susceptibility