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We have previously demonstrated the detection of reversible and irreversible changes on MR images oflaser energy
deposition and tissue heating and cooling1. It is possible to monitor and control energy deposition during interstitial laser
therapy. This presentation describes some first steps toward optimizing the power and total energy deposited in various
tissues in vivo, by analyzing the irreversible tissue changes and their spatial distribution as revealed by spin echo imaging.
We used various power settings of an Nd.YAG laser delivered by a fiber optic inserted into several tissues (brain, muscle,
liver) of anesthetized rats and rabbits. MR imaging was performed at 1.9 T. Photothermally-produced lesions were seen on
both T1- and Ta-weighted images. The overall size of the lesions correlated with the magnitude of the energy applied. The
MR image appearance depended not only on the laser energy but also on the way it was delivered, on the type of tissue, and
the MR pulse sequence applied. While Ti-weighted images adequately demonstrated an area of tissue destruction, T2-
weighted images showed a more heterogeneous and more extensive lesion which could be better correlated with the complex
histological representation of these lesions. Typically, when rabbit brain, liver, and muscle had been exposed to laser power
of 2.5 Watts for a range of 55 to 120 seconds, depending on the tissue, a central area of signal void was surrounded by an
inner hypointensity and an outer hyperintensity on T2-weighted images. The 3D extent of the lesions was well
demonstrated on multislice images, providing correlation of the affected volumes seen on MRI with volumes seen in
histological or histochemical preparations. We are developing an analytical model of laser heating and its effect on MR
images to assess whether heating during imaging will produce unacceptable artifacts during surgery. The effect of heating is
modeled as a change in magnetization during image acquisition. The region in which the change occurs is blurred by the
Fourier transform of the change in magnetization as a function of time. Thus, blurring is minimized when changes occur
slowly, compared to image acquisition times. We conclude that MRI can demonstrate the 3D extent of the lesions induced
by lasers and can be used to investigate and optimize the control of induced tissue change within the affected volume.
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