Kathleen M. Brennan

Temperature determination in the frozen region during cryosurgery of rabbit liver using MR image analysis

Cryosurgery currently is being used clinically to treat tumors in internal organs such as the liver and prostate. Although performed at present under ultrasound monitoring, magnetic resonance imaging (MRI)-guidance of these procedures not only permits monitoring of the frozen region during cryosurgery but also makes it possible to determine the temperature distribution in the frozen region, which is not possible using ultrasound monitoring. A good estimate of the region of destruction in the tissue can be obtained from correlating the temperature distribution and the time course of the freezing with the image of the frozen region. Unfortunately, MR pulse sequence-based temperature determination techniques such as diffusion, relaxation time, and chemical shift cannot be used for measuring the temperature in the frozen region because the T2 of the frozen regions is so short that there is effectively no RF signal from the frozen region. This paper describes a numerical technique for determining the two dimensional temperature distribution in the frozen region during MR image-guided cryosurgery of normal liver in rabbits. The technique involves solving the energy equation numerically in the frozen region to determine the temperature distribution there. The boundary conditions needed to solve the equation are determined from MR images of the frozen tissue during cryosurgery and from the measured temperature of the cryoprobe. The calculated temperature in the frozen region is then correlated with the damaged region (cryolesion) determined from post mortem histologic evaluation.

MRI-monitored cryosurgery in the rabbit brain.

The inability to observe the transient, irregular shape of the frozen region that develops during cryosurgery has inhibited the application of this surgical technique to the treatment of tumors in the brain and deep in visceral organs. We used proton NMR spin-echo and spoiled gradient-echo imaging to monitor the development of frozen lesions during cryosurgery in the rabbit brain and the resulting postcryosurgical changes up to 4 hr after freezing. Spoiled gradient-echo images (TE = 14 ms; TR = 50 ms) were acquired during freezing and thawing at a rate of 15 s/slice. Although the frozen region itself is invisible in MR images, its presence is distinguished easily from the surrounding unfrozen soft tissue because of the large contrast difference between frozen and unfrozen regions. T2-weighted spin-echo images (TE = 100 ms, TR = 2 s) obtained after thawing suggest that edema forms first at the margin of the region that was frozen (cryolesion) and then moves into the regions core. Histological examination showed complete necrosis in the cryolesion and a sharp transition to undamaged tissue at the margin of the lesion and its image. Blood-brain barrier (BBB) damage was investigated using gadolinium-DTPA. The region of edema in the T2-weighted spin-echo images was coincident with the area of BBB damage in the Gd-DTPA-enhanced T1-weighted spin-echo images (TE = 33 ms, TR = 400 ms) and both were distinguishable as areas of high signal relative to the surrounding normal tissue. The results of these experiments indicate that MR can both effectively monitor the cryosurgical freezing and thawing cycle and characterize the postcryosurgical changes in tissue during follow-up.

In vivo imaging of the 5-hydroxytryptamine reuptake site in primate brain using single photon emission computed tomography and [123I]5-iodo-6-nitroquipazine

Previous experiments have demonstrated that 5-iodo-6-nitro-2-piperazinylquinoline (5-I-6-NQP) is a potent and selective ligand for studying brain 5-hydroxytryptamine (5-HT) reuptake sites. We performed in vivo imaging in non-human primates using single photon emission computed tomography (SPECT) and the 123I-labeled compound [123I]5-I-6-NQP. These studies showed rapid brain uptake, with slow egress of the tracer from the brainstem, a region rich in 5-HT reuptake sites. Loss of the tracer from regions with a lower density of these sites, such as cerebellum, was relatively more rapid. Pretreatment of animals with paroxetine increased the washout of tracer from the brainstem to rates similar to that seen in cerebellum. Brainstem to cerebellar ratios of tracer accumulation were > 2 by 8 h after injection, and in paroxetine pretreated animals remained close to 1. These results indicate that the radiotracer has characteristics suitable for use as a SPECT imaging agent of serotonin reuptake sites.

MRI and PET of delayed heavy-ion radiation injury in the rabbit brain

Magnetic resonance imaging (MRI) and positron emission tomography (PET) techniques were used to obtain in vivo scans of delayed (30 GyE helium ion, 230 MeV/u) radiation injury in rabbit brain. T2-weighted (T2W) MRI scans demonstrated alterations that were restricted primarily to the white matter tracts and the deep perithalamic and thalamic regions. Quantitative measurements of T2 and T1 values demonstrated wide variations in absolute values. However, paired comparisons in hemibrain-irradiated rabbits revealed significant increases in T2 (p less than 0.001) and T1 (p less than 0.01) in irradiated versus unirradiated brain. Gadolinium DTPA (GdDTPA) enhanced MRI and 82Rubidium (82Rb) PET detected focal regions of blood-brain barrier (BBB) disruption restricted to the deep white matter and thalamic regions. Sequential GdDTPA enhanced MRI scans showed the spreading of the tracer from the initial site of contrast enhancement. 18Fluorodeoxyglucose (18FDG) PET studies demonstrated the markedly depressed metabolic profiles of irradiated brain. Histological findings of tissue edema and necrosis correlated well with the in vivo imaging abnormalities. These initial studies demonstrate that the irradiated rabbit brain is a suitable animal model for examining the delayed effects of radiation injury in the brain.

PET studies of cerebral glucose metabolism in conscious rhesus macaques

A growing body of evidence suggests that rhesus macaques may be a good model of human brain aging. We used positron emission tomography (PET) and 18F-fluorodeoxyglucose (FDG) to measure regional cerebral metabolic rates for glucose (rCMRglc) in young and aged rhesus macaques to determine if age-related decreases, such as those reported in humans, also occur in macaques. Whereas the aged animals had lower metabolic rates in every brain region studied, the largest differences were in left temporal cortex. The largest differences were also observed in left temporal cortex when relative rCMRglc values were used. Both rCMRglc and relative rCMRglc were marked by substantial individual variation within the aged group. This variation may parallel the variation observed in behavioral studies. Future studies that include both PET and behavioral measures should help determine if there is a relationship between age-related changes in rCMRglc and behavior.

A study of radiation necrosis and edema in the canine brain using positron emission tomography and magnetic resonance imaging.

Radiation injury, a major hazard of central nervous system (CNS) radiotherapy, was investigated using sequential studies with positron emission tomography (PET) and magnetic resonance imaging (MRI) in beagle dogs with both helium and neon-ion hemibrain irradiation. All dogs receiving 7.5-11 Gy of neon showed no signs of radiation injury to 3 years after irradiation. Dogs receiving > or = 13 Gy neon or helium succumbed to radiation necrosis and died 21-32 weeks after irradiation. The findings of imaging studies for all dogs who succumbed to radiation necrosis were normal until 3-6 weeks before death. Sequential studies were performed using 0.5 T MRI spin-echo and inversion recovery imaging sequences, and high-resolution (2-3 mm) PET with 18F deoxyglucose and 82Rb. The same axial slices (within 1-2 mm) were imaged repeatedly (weekly) after irradiation until death. The earliest CNS changes were seen as decreased metabolic activity in the cortex of the irradiated hemisphere with PET or an increase in signal intensity in the periventricular white matter on T2-weighted spin-echo imaging on MRI. From the time this increase in signal intensity was first observed, T1 and T2 values increased steadily in both the gray and white matter until death. The changes in white matter were consistently greater than those in gray matter. The results of PET, MRI, and histopathological examinations support the theory that both cellular and vascular mechanisms are involved in radiation necrosis.

Advances in positron tomography for oncology

Development of PET instrumentation over the past 42 years has moved from simple dual-detector coincidence scanners, to proposed systems having 60,000 detectors and simultaneous coverage of 15-cm regions of the body with spatial resolutions better than 4 mm. The principal determinants of positron emission tomography (PET) instrumentation advances are positron range, noncollinearity of the annihilation photons, scattering, random event rates, detector size, efficiency, speed and light output; capability to correct for depth of crystal interaction, attenuation compensation, axial coverage, and rapid data analysis and presentation. While general-purpose systems with 2-mm resolution are expected, special-purpose PET devices are being built for breast and brain tumor studies with resolutions from 1.7 to 5 mm.

Cerebrovascular and metabolic perturbations in delayed heavy charged particle radiation injury.

Focal heavy charged particle irradiation of the rabbit brain created defined lesions which were observable by nuclear magnetic resonance (NMR) and positron emission tomography (PET) imaging techniques. The lesions appeared approximately 9-11 months after left partial hemibrain irradiation with 30 Gy (230 MeV/u helium ions), and were restricted to the white matter tracts and deep perithalamic and thalamic regions. 82Rubidium PET and Gadolinium DTPA enhanced NMR imaging were used to detect blood-brain barrier perturbations. 18Fluordeoxyglucose PET studies demonstrated widespread decreases in cerebral glucose uptake in the cortex and thalamus of the irradiated hemisphere. NMR and PET imaging results correlated well with histological findings. Rabbits irradiated with 15 Gy did not demonstrate any abnormalities in the brain with sequential NMR scans through 14 months post-irradiation.

Longitudinal Evaluation of Left Ventricular Substrate Metabolism, Perfusion, and Dysfunction in the Spontaneously Hypertensive Rat Model of Hypertrophy Using Small-Animal PET/CT Imaging

Myocardial metabolic and perfusion imaging is a vital tool for understanding the physiologic consequences of heart failure. We used PET imaging to examine the longitudinal kinetics of 18F-FDG and 14(R,S)-18F-fluoro-6-thia-heptadecanoic acid (18F-FTHA) as analogs of glucose and fatty acid (FA) to quantify metabolic substrate shifts with the spontaneously hypertensive rat (SHR) as a model of left ventricular hypertrophy (LVH) and failure. Myocardial perfusion and left ventricular function were also investigated using a newly developed radiotracer 18F-fluorodihydrorotenol (18F-FDHROL). Methods: Longitudinal dynamic electrocardiogram-gated small-animal PET/CT studies were performed with 8 SHR and 8 normotensive Wistar-Kyoto (WKY) rats over their life cycle. We determined the myocardial influx rate constant for 18F-FDG and 18F-FTHA (KiFDG and KiFTHA, respectively) and the wash-in rate constant for 18F-FDHROL (K1FDHROL). 18F-FDHROL data were also used to quantify left ventricular ejection fraction (LVEF) and end-diastolic volume (EDV). Blood samples were drawn to independently measure plasma concentrations of glucose, insulin, and free fatty acids (FFAs). Results: KiFDG and KiFTHA were higher in SHRs than WKY rats (P < 3 × 10−8 and 0.005, respectively) independent of age. A decrease in KiFDG with age was evident when models were combined (P = 0.034). The SHR exhibited higher K1FDHROL (P < 5 × 10−6) than the control, with no age-dependent trends in either model (P = 0.058). Glucose plasma concentrations were lower in SHRs than controls (P < 6 × 10−12), with an age-dependent rise for WKY rats (P < 2 × 10−5). Insulin plasma concentrations were higher in SHRs than controls (P < 3 × 10−3), with an age-dependent decrease when models were combined (P = 0.046). FFA levels were similar between models (P = 0.374), but an increase with age was evident only in SHR (P < 7 × 10−6). Conclusion: The SHR exhibited alterations in myocardial substrate use at 8 mo characterized by increased glucose and FA utilizations. At 20 mo, the SHR had LVH characterized by decreased LVEF and increased EDV, while simultaneously sustaining higher glucose and similar FA utilizations (compared with WKY rats), which indicates maladaptation of energy substrates in the failing heart. Elevated K1FDHROL in the SHR may reflect elevated oxygen consumption and decreased capillary density in the hypertrophied heart. From our findings, metabolic changes appear to precede mechanical changes of LVH progression in the SHR model.

Morphological and kinetic abnormalities of platelets in hypercholesterolemic rabbits

Hypercholesterolemia (HC = hypercholesterolemia or hypercholesterolemic) was produced in rabbits by feeding them diets supplemented with cholesterol and peanut oil. Platelet counts and volumes, white cell counts, reticulocyte counts, and hematocrits were determined at intervals for 8-12 weeks in blood from HC animals and controls on a normal rabbit diet. Microthrombocytosis was a consistent occurrence in the presence of HC, developing as early as 2 weeks into the diet. Microthrombocytosis was generally associated with normal platelet counts, but mild thrombocytosis occurred late in the diet at the time of the highest levels of serum cholesterol (greater than 1300 mg/dl). Platelets from HC rabbits were morphologically normal by transmission electron microscopy. Survivals of 51Cr-labeled platelets from HC and non-HC rabbits were measured in HC and non-HC recipients. The results identified an intrinsic defect in the ability of HC platelets to survive in the circulation. They also confirmed previous findings of an environmental defect in HC that causes shortened platelet survival.