Nobuya Higuchi

Biexponential apparent diffusion coefficients in prostate cancer

PURPOSE The purpose of this study was to investigate the need for biexponential signal decay modeling for prostate cancer diffusion signal decays with b-factor over an extended b-factor range. MATERIALS AND METHODS Ten healthy volunteers and 12 patients with a bulky prostate cancer underwent line scan diffusion-weighted MR imaging in which b-factors from 0 to 3000 s/mm(2) in 16 steps were sampled. The acquired signal decay curves were fit with both monoexponential and biexponential signal decay functions and a statistical comparison between the two fits was performed. RESULTS The biexponential model provided a statistically better fit over the monoexponential model on the peripheral zone (PZ), transitional zone (TZ) and prostate cancer. The fast and slow apparent diffusion coefficients (ADCs) in the PZ, TZ and cancer were 2.9+/-0.2, 0.7+/-0.2 x 10(-3) mm(2)/ms (PZ); 2.9+/-0.4, 0.7+/-0.2 x 10(-3) mm(2)/ms (TZ); and 1.7+/-0.4, 0.3+/-0.1 x 10(-3) mm(2)/ms (cancer), respectively. The apparent fractions of the fast diffusion component in the PZ, TZ and cancer were 70+/-10%, 60+/-10% and 50+/-10%, respectively. The fast and slow ADCs of cancer were significantly lower than those of TZ and PZ, and the apparent fraction of the fast diffusion component was significantly smaller in cancer than in PZ. CONCLUSIONS Biexponential diffusion decay functions are required for prostate cancer diffusion signal decay curves when sampled over an extended b-factor range, providing additional, unique tissue characterization parameters for prostate cancer.

Magnetic resonance imaging of the acute effects of interstitial neodymium:YAG laser irradiation on tissues.

RATIONALE AND OBJECTIVES.Laser irradiation therapy in deep tissues requires a monitoring method other than visual guidance. Magnetic resonance imaging (MRI) can be used for this purpose because it visualizes soft tissue structures and heat distribution. METHODS.The authors performed interstitial laser irradiations in rat livers with various laser outputs and measured the sizes of laser-induced lesions. MRI of these lesions was done exvivo and compared with the histologic findings. Laser-induced lesions also were studied in rabbit brain, liver, and skeletal muscle to show the influences of tissue optical and thermal properties. Imaging of interstitial laser irradiation also was performed in vivo in rabbit brains. RESULTS.MRI depicted the laser-induced lesions produced with different laser outputs and tissue types. MRIs of rabbit brain in vivo effectively demonstrated the signal decrease during heating and acute tissue changes. CONCLUSION.MRI has potential for monitoring interstitial laser surgery or hyperthermia.

Response to and Control of Destructive Energy by Magnetic Resonance

Magnetic resonance imaging techniques can be used to control and monitor the deposition of destructive energy. The authors evaluated the feasibility of phosphorus-31 magnetic resonance spectroscopy for the control, monitoring, and prediction of the three-dimensional extent of tissue destruction during interstitial laser surgery. Characteristic metabolic changes were demonstrated within the lesion and in the adjacent normal tissue during the deposition of thermal energy.

Magnetic resonance imaging of interstitial laser photocoagulation

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.

Magnetic resonance imaging and spectroscopy to monitor and control experimental laser surgery

We have proposed the use of MRI for monitoring and control of interstitial laser surgery, in order to improve the accuracy and reduce the invasiveness of these procedures. To expand the knowledge base about the MR appearance of laser-induced tissue damage, we applied MR imaging and phosphorus-31 MR spectroscopy to detect the changes induced in various tissues by radiation from an Nd:YAG laser at 1060 nm wavelength delivered interstitially through a fiber optic waveguide. A range of laser energies was applied, and laser pulse parameters were varied. Proton MR images of the laser-produced lesions were compared with the histological appearance in brain and liver tissue of experimental animals. The spatial extent of laser effects differed among tissue types, and this was well reflected on MR images. The distribution of MR signal change resulting from different laser exposures was also demonstrated. Experimental laser surgery was performed in animal brain and bladder. Images taken before, during, and after laser irradiation allowed us to distinguish between reversible thermal and permanent effects. This information was utilized to tailor the destruction of preselected targets while minimizing damage to surrounding tissues. Qualitative changes were also revealed on phosphorus spectra. Irreversible lesions were characterized by overall line broadening and a decrease in AT?. There was also a large relative increase in the inorganic phosphate region of the spectrum. These demonstrations are a big step toward achieving our ultimate goal, the development of MR-controlled laser surgery.