Andriy Babsky

Noninvasive MR thermometry using paramagnetic lanthanide complexes of 1,4,7,10-tetraazacyclodoecane-α, α', α , α' -tetramethyl-1,4,7,10-tetraacetic acid (DOTMA4-)

Noninvasive techniques to monitor temperature have numerous useful biomedical applications. However, MR thermometry techniques based on the chemical shift, relaxation rates, and molecular diffusion rate of the water 1H signal suffer from poor thermal resolution. The feasibility of MR thermometry based on the strong temperature dependence of the hyperfine‐shifted 1H signal from the paramagnetic lanthanide complex thulium‐1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetate (TmDOTA–) was recently demonstrated. The use of paramagnetic lanthanide complexes for MR thermometry can be further enhanced by improving the signal‐to‐noise ratio (SNR) of the observed signal. In this study, the use of lanthanide complexes of a methyl‐substituted analog of DOTA4–, 1,4,7,10‐tetramethyl 1,4,7,10‐tetra azacyclodoecane‐1,4,7,10‐tetraacetic acetate (DOTMA4–) was evaluated. DOTMA4– complexes have 12 magnetically equivalent methyl protons, which provide an intense and sharper resonance compared to the corresponding DOTA– complexes. Experiments with paramagnetic Pr3+, Yb3+, Tb3+, Dy3+, and Tm3+ complexes of DOTMA4– showed that the Tm3+ complex is most favorable for MR thermometery because of the high temperature dependence of its chemical shift and its relatively narrow linewidth. The chemical shift of the methyl 1H signal from TmDOTMA– was ∼60 times more sensitive to temperature than the water 1H shift and was insensitive to changes in concentration, pH, [Ca2+], or the presence of other ions and macromolecules. The application of TmDOTMA– for measuring temperature in a subcutaneously implanted tumor model was demonstrated. Lastly, the feasibility of obtaining 3D images from the methyl 1H resonance of TmDOTMA– was demonstrated in phantom and live animal experiments. Overall, TmDOTMA– appears to be a promising probe for MR thermometry in vivo. Magn Reson Med 53:294–303, 2005.

Evaluation of extra‐ and intracellular apparent diffusion coefficient of sodium in rat skeletal muscle: Effects of prolonged ischemia

The mechanism of water and sodium apparent diffusion coefficient (ADC) changes in rat skeletal muscle during global ischemia was examined by in vivo 1H and 23Na magnetic resonance spectroscopy (MRS). The ADCs of Na+ and water are expected to have similar characteristics because sodium is present as an aqua‐cation in tissue. The shift reagent, TmDOTP5‐, was used to separate intra‐ and extracellular sodium (Na  i+ and Na  e+ , respectively) signals. Water, total tissue sodium (Na  t+ ), Na  i+ , and Na  e+ ADCs were measured before and 1, 2, 3, and 4 hr after ischemia. Contrary to the general perception, Na  i+ and Na  e+ ADCs were identical before ischemia. Thus, ischemia‐induced changes in Na  t+ ADC cannot be explained by a simple change in the size of relative intracellular or extracellular space. Na  t+ and Na  e+ ADCs decreased after 2–4 hr of ischemia, while water and Na  i+ ADC remained unchanged. The correlation between Na  t+ and Na  e+ ADCs was observed because of high Na  e+ concentration. Similarly, the correlation between water and Na  i+ ADCs was observed because cells occupy 80% of the tissue space in the skeletal muscle. Ischemia also caused an increase in the Na  i+ and an equal decrease in Na  e+ signal intensity due to cessation of Na+/K+‐ATPase function. Magn Reson Med 59:485–491, 2008.

Early monitoring of acute tubular necrosis in the rat kidney by 23Na-MRI

Reabsorption of water and other molecules is dependent on the corticomedullary sodium concentration gradient in the kidney. During the early course of acute tubular necrosis (ATN), this gradient is altered. Therefore, 23Na magnetic resonance imaging (MRI) was used to study the alterations in renal sodium distribution in the rat kidney during ischemia and reperfusion (IR) injury, which induces ATN. In-magnet ischemia was induced for 0 (control), 10, 20, 30 or 50 min in Wistar rats. 23Na images were collected every 10 min during baseline, ischemia, and 60-min reperfusion periods. T1 and T2 relaxation times were measured by both 23Na-MRI and -MRS on a separate cohort of animals during ischemia and reperfusion for correction of relaxation-related tissue sodium concentration (TSC). A marked decrease was observed in the medulla and cortex 23Na-MRI signal intensity (SI) during the early evolution of ATN caused by IR injury, with the sodium reabsorption function of the kidney being irreversibly damaged after 50 min of ischemia. Sodium relaxation time characteristics were similar in the medulla and cortex of normal kidney, but significantly decreased with IR. The changes in relaxation times in both compartments were identical; thus the medulla-to-cortex sodium SI ratio represents the TSC ratio of both compartments. The extent of IR damage observed with histological examination correlated with the 23Na-MRI data. 23Na-MRI has great potential for noninvasive, clinical diagnosis of evolving ATN in the setup of acute renal failure and in differentiating ATN from other causes of renal failure where tubular function is maintained.

Controlled radio-frequency hyperthermia using an MR scanner and simultaneous monitoring of temperature and therapy response by 1H, 23Na and 31P magnetic resonance spectroscopy in subcutaneously implanted 9L-gliosarcoma

A magnetic resonance (MR) technique is developed to produce controlled radio-frequency (RF) hyperthermia (HT) in subcutaneously-implanted 9L-gliosarcoma in Fisher rats using an MR scanner and its components; the scanner is also simultaneously used to monitor the tumour temperature and the metabolic response of the tumour to the therapy. The method uses the 1H chemical shift of thulium 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetra-acetic acid (TmDOTA−) to monitor temperature. The desired HT temperature is achieved and maintained using a feedback loop mechanism that uses a proportional-integral-derivative controller. The RF HT technique was able to heat the tumour from 33° to 45°C in ∼10 min and was able to maintain the tumour temperature within ±0.2°C of the target temperature (45°C). Simultaneous monitoring of the metabolic changes with RF HT showed increases in total tissue and intracellular Na+ as measured by single-quantum and triple-quantum filtered 23Na MR spectroscopy (MRS), respectively, and decreases in intra- and extracellular pH and cellular bioenergetics as measured by 31P MRS. Monitoring of metabolic response in addition to the tumour temperature measurements may serve as a more reliable and early indicator of therapy response. In addition, such measurements during HT treatment will enhance our understanding of the tumour response mechanisms during HT, which may prove valuable in designing methods to improve therapeutic efficiency.

Absolute temperature MR imaging with thulium 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetramethyl-1,4,7,10-tetraacetic acid (TmDOTMA−)

MR thermometry based on the water 1H signal provides high temporal and spatial resolution, but it has low temperature sensitivity (∼0.01 ppm/°C) and requires monitoring of another weaker signal for absolute temperature measurements. The use of the paramagnetic lanthanide complex, thulium 1,4,7,10‐ tetraazacyclo‐dodecane‐1,4,7,10‐tetramethyl‐1,4,7,10‐tetraac‐ etate (TmDOTMA−), which is ∼60 times more sensitive to temperature than the water 1H signal, is advanced to image absolute temperatures in vivo using water signal as a reference. The temperature imaging technique was developed using gradient echo and asymmetric spin echo imaging sequences on 9.4 Tesla (T) horizontal and vertical MR scanners. A comparison of regional temperatures measured with TmDOTMA− and fiber‐optic probes showed that the accuracy of imaging temperature is <0.3°C. The temperature imaging technique was found to be insensitive to inhomogeneities in the main magnetic field. The feasibility of imaging temperature of intact rats at ∼1.4 mmol/kg dose with ∼1‐mm spatial resolution in only 3 min is demonstrated. TmDOTMA− should prove useful for imaging absolute temperatures in deep‐seated organs in numerous biomedical applications. Magn Reson Med, 2009.

Heat-induced changes in intracellular Na+, pH and bioenergetic status in superfused RIF-1 tumour cells determined by 23Na and 31P magnetic resonance spectroscopy

The acute effects of hyperthermia on intracellular Na+ (Na), bioenergetic status and intracellular pH (pHi) were investigated in superfused Radiation Induced Fibrosarcoma–1 (RIF-1) tumour cells using shift-reagent-aided 23Na and 31P nuclear magnetic resonance (NMR) spectroscopy. Hyperthermia at 45°C for 30 min produced a 50% increase in Na, a 0.42 unit decrease in pHi and a 40–45% decrease in NTP/Pi. During post-hyperthermia superfusion at 37°C, pHi and NTP/Pi recovered to the baseline value, but Na initially decreased and then increased to the hyperthermic level 60 min after heating. Hyperthermia at 42°C caused only a 15–20% increase in Na. In the presence of 3 µM 5-(N-ethyl-N-isopropyl)amiloride (EIPA), an inhibitor of the Na+/H+ exchanger, the increase in Na during 45°C hyperthermia was attenuated, suggesting that the heat-induced increase in Na was mainly due to an increase in Na+/H+ anti-porter activity. EIPA did not prevent hyperthermia-induced acidification. This suggests that pHi is controlled by other ion exchange mechanisms in addition to the Na+/H+ exchanger. EIPA increased the thermo-sensitivity of the RIF-1 tumour cells only slightly as measured by cell viability and clonogenic assays. The hyperthermia-induced irreversible increase in Na suggests that changes in transmembrane ion gradients play an important role in cell damage induced by hyperthermia.

Fat and Water 1H MRI to Investigate Effects of Leptin in Obese Mice

Leptin is known to be associated with regulation of body weight and fat content. The effects of exogenous leptin on abdominal visceral (VS) and subcutaneous (SC) fat volume and hepatic fat‐to‐water ratio in leptin‐deficient obese mice were investigated by 1H magnetic resonance imaging (MRI). Chemical shift‐selected fat and water 1H MRI of control and leptin‐treated mice were obtained 1 day before treatment and after 7 days of treatment (0.3 mg/kg/day). Hepatic fat‐to‐water ratio and VS fat volume decreased significantly with treatment, whereas SC fat volume did not change. Noninvasive measurement of fat and water content in different body regions using MRI should prove useful for evaluating new drugs for the treatment of obesity and other metabolic disorders.

Effect of implantation site and growth of hepatocellular carcinoma on apparent diffusion coefficient of water and sodium MRI

Hepatocellular carcinoma (HCC) and liver metastases are an increasing problem worldwide. Non‐invasive methods for the early detection of HCC and understanding of the tumor growth mechanisms are highly desirable. Both the diffusion‐weighted 1H (DWI) and 23Na MRI reflect alterations in tissue compartment volumes in tumors, as well as physiological and metabolic transformation in cells. Effects of untreated growth on apparent diffusion coefficient of water (ADC), single quantum (SQ) and triple quantum‐filtered (TQF) 23Na MRI were compared in intrahepatically and subcutaneously implanted HCCs in rats. Animals were examined weekly for 4 weeks after injection of N1S1 cells. ADC of intrahepatic HCC was 1.5‐times higher compared to the nearby liver tissue, and with growth, the ADC did not increase. ADC of subcutaneous HCC was lower compared to intrahepatic HCC and it increased with growth. Untreated growth of both intrahepatic and subcutaneous HCCs was associated with an increase in SQ and TQF 23Na signal intensity suggesting an increase in tissue Na+ and intracellular Na+ (Na+i), respectively, most likely due to an increase in relative extracellular space and Na+i concentration as a result of changes in tissue structure and cellular metabolism. Thus, SQ and TQF 23Na MRI may be complementary to diffusion imaging in areas susceptible to motion for characterizing hepatic tumors and for other applications, such as, predicting and monitoring therapy response. Copyright

Variability of apoptosis and response in N1-S1 rodent hepatomas to benzamide riboside and correlation to early changes in water apparent diffusion coefficient and sodium MR imaging

PURPOSE This pilot trial assesses variability of apoptosis and response 1 day after hepatic intraarterial (IA) benzamide riboside (BR) in rodent hepatomas and its correlation to water apparent diffusion coefficient (ADC) and single-quantum (SQ) and triple-quantum-filtered (TQF) sodium-23 ((23)Na) magnetic resonance (MR) imaging. MATERIALS AND METHODS Sprague-Dawley rats (n = 8) were inoculated with 10(6) N1-S1 cells. IA BR (20 mg/kg) was infused after 14 days. Animals were killed 1 day (n = 4) or 21 days (n = 4) after therapy. Imaging was performed 1 day before and after treatment. Volume was assessed over 2 weeks. Percentage apoptosis was counted from terminal deoxynucleotidyl transferase dUTP nick-end labeling-stained slides at 400×magnification. Kruskal-Wallis tests were used to compare apoptosis, and Wilcoxon signed-rank tests were used to compare MR signal intensity (SI). RESULTS Apoptosis was marginally greater in tumor than in nontumor (6.7% vs 1.3%; P = .08), varying from 2% to 10%. Before treatment, MR SI was greater in tumor than in nontumor (ADC, 1.18 vs 0.76 [P = .0078]; SQ, 1.20 vs 1.04 [P = .03]; TQF, 0.55 vs 0.34 [P = .03]). After treatment, tumors increased in volume (0.62 vs 0.33; P = .016) variably over 2 weeks. MR SI remained greater in tumor than in nontumor (ADC, 1.20 vs 0.77 [P = .0078]; SQ, 1.76 vs 1.15 [P = .016]; TQF, 0.84 vs 0.49 [P = .03]). SQ and TQF SI increased by 47% (P = .016) and 53% (P = .016) in tumors, whereas ADC did not change. CONCLUSIONS Apoptosis was marginal and varied from 2% to 10%. Water ADC, SQ, and TQF MR imaging distinguished tumor from nontumor. Changes in water ADC and sodium MR imaging correlated to apoptosis and volume in select cases, but additional animals are needed to validate this trend against tumor growth.