New polyvinyl alcohol gel material for MRI phantoms

Investigation and analysis of ferrous sulfate polyvinyl alcohol (PVA) gel dosimeter

Ferrous sulfate (Fe(SO4)2) PVA gels were investigated for a range of absorbed doses up to 20 Gy using both magnetic resonance imaging (MRI) and spectrophotometry to determine R1 and optical density (OD) dose responses and G values. It was found that R1- and OD-dose sensitivities increased with O2 saturation or by the introduction of a freeze-thaw cycle during preparation of the PVA gel. The storage temperature of the Fe(SO4)2 PVA gel at -18 degrees C increased R1-dose sensitivity above that of gels stored at 5 degrees C. The addition of sucrose to the formulation was found to result in the largest increase in both R1- and OD-dose sensitivities. Fe(SO4)2 PVA gel with and without the addition of xylenol orange was demonstrated to have a G value of approximately 20 ions/100 eV and with sucrose approximately 24 ions/100 eV.

Comparison of core needle breast biopsy techniques: freehand versus three-dimensional US guidance

RATIONALE AND OBJECTIVES

No single method is generally accepted for evaluating the accuracy of breast biopsy techniques before their clinical implementation. The purpose of this study was to test a new process for evaluating biopsy techniques by using it in the evaluation of a prototype three-dimensional ultrasound (US)-guided biopsy device.

MATERIALS AND METHODS

The biopsy accuracy of a new three-dimensional US-guided breast biopsy device was compared to that of the accepted clinical practice of biopsy by expert radiologists with two-dimensional freehand US guidance. Biopsies were performed in chicken tissue phantoms containing 3.2-mm lesions made of poly(vinyl alcohol) cryogel. The criterion for a successful biopsy was the presence of lesion in the sample. The equivalence limit difference tested was 10% by using a power of 90% and a two-sided test significance level, a, of 10%.

RESULTS

The biopsy success rate of the three-dimensional US-guided system (96%) was equivalent to that of expert radiologists using two-dimensional freehand US guidance (94.5%) in tissue phantoms containing poly(vinyl alcohol) cryogel lesions.

CONCLUSION

This evaluation procedure is a valuable precursor to clinical trials in the assessment of biopsy techniques. The three-dimensional US-guided breast biopsy system provides a suitable alternative to two-dimensional freehand US guidance for biopsy of breast cancer.

Temperature dosimetry using MR relaxation characteristics of poly(vinyl alcohol) cryogel (PVA-C)

Hyperthermic therapy is being used for a variety of medical treatments, such as tumor ablation and the enhancement of radiation therapy. Research in this area requires a tool to record the temperature distribution created by a heat source, similar to the dosimetry gels used in radiation therapy to record dose distribution. Poly(vinyl alcohol) cryogel (PVA-C) is presented as a material capable of recording temperature distributions between 45 and 70 degrees C, with less than a 1 degrees C error. An approximately linear, positive relationship between MR relaxation times and applied temperature is demonstrated, with a maximum of 16.3 ms/ degrees C change in T(1) and 10.2 ms/ degrees C in T(2) for a typical PVA-C gel. Applied heat reduces the amount of cross-linking in PVA-C, which is responsible for a predictable change in T(1) and T(2) times. Temperature distributions in PVA-C volumes may be determined by matching MR relaxation times across the volumes to calibration values produced in samples subjected to known temperatures. Factors such as thermotolerance, perfusion effects, and thermal conductivity of PVA-C are addressed for potentially extending this method to modeling thermal doses in tissue.

Elucidation of accuracy in calibration of MR signal intensity based on transmission amplitude method

To calibrate magnetic resonance (MR) signal intensity that depends on radio frequency (RF) coil loading, the transmission amplitude (TRA) for the excitation in the transmit-receive RF coil has been used as a good index in the so-called TRA method. As this TRA method needs neither an internal reference nor an additional external reference for the calibration, its accuracy is free from reference measurements. This study elucidated the calibration accuracy of MR signal intensities based on the TRA method. A cylindrical gel phantom was used for accuracy measurements with a 1.5-T MRI unit with conventional T1 imaging as a simple pulse sequence for various loading conditions. The brain parenchyma of eight healthy volunteers also showed calibrated MR signal deviations. The error of the phantom calibration measurements was 2.18% (S.D.%). The background noise intensity of images was theoretically derived to correlate with the impedance mismatching of the RF coil, which is inevitable for fixed tuning, even for automatic tuning that is not always exact. Taking into account this noise intensity, the calibration method was modified to reduce its error to 1.50%. The standard deviations of the calibrated values in the thalamus and frontal white matter were 2.9 and 3.8%, respectively. We suggest that the modified TRA method is a practical and reliable technique to obtain clinical numeric evidence.

Heavily T1-weighted images without respiratory artifacts: partial angle inversion recovery fast spin-echo imaging (PAIR-FSE)

We obtained T1-weighted images in the abdominal region using the partial angle inversion recovery fast spin echo (PAIR-FSE) with the respiratory triggering (RT) method and compared the image quality with that of conventional SE (TR/TE 500/10 msec) with the partial angle inversion recovery (PEAR) method. The signal difference to noise ratio of the PAIR-FSE was 1.6 times higher (6.94 +/- 3.08) than that of SE (4.30 +/- 1.88). Respiratory motion-induced ghost artifacts were reduced by half in PAIR-FSE with RT (1.01 +/- 0.47) in comparison with SE with PEAR (2.24 +/- 0.70). J. Magn. Reson. Imaging 2000;12:960-964.

Volumetric measurement of renal cysts and parenchyma using MRI: phantoms and patients with polycystic kidney disease

We have developed an MR method to measure the volumes of renal cysts and parenchyma in patients with polycystic kidney disease. Phantoms were designed to simulate polycystic kidneys. Four patients were recruited. MR scans were performed on the phantoms and patients. A stereology technique was applied for image segmentation and volume measurement. Volumetric measurement of renal cysts and parenchyma was accurate in phantom studies and reliable in both phantom and patient studies in these limited examples.

Polyvinyl alcohol-Fricke hydrogel and cryogel: two new gel dosimetry systems with low Fe3+ diffusion

Two new Fricke dosimeter gel systems with low diffusion rates have been developed for 3D radiation dosimetry purposes. Both systems consist of a solution of 20% (by weight) polyvinyl alcohol (PVA) in a 50 mM H2SO4 solution with 0.4 mM ferrous ammonium sulphate and xylenol orange (FX). The difference in the two gels is the way that the gelation process was initiated: either by bringing the temperature to (a) +5 degrees C or (b) -20 degrees C before returning them to room temperature. These gels are termed 'hydrogel' and 'cryogel', respectively. The hydrogel is optically transparent, and can be used with either optical or MRI detection methods for dosimetric imaging. The cryogel is rubbery in texture but opaque, so its internal Fe3+ concentration can only be measured with MRI. The hydrogel's optical attenuation coefficient is linear (r2 = 0.99) with dose from 0 to 20 Gy with a sensitivity of 0.106 cm(-1) Gy(-1) (at 543 nm). In terms of MR relaxation rate, the dose response for both the hydrogel and cryogel was linear (r2 = 0.99) with a sensitivity of 0.020 s(-1) Gy(-1) (at 1.5 T). The Fe3+ diffusion coefficient (at 20 degrees C) was measured to be 0.14 mm2 h(-1), which is significantly lower than similar preparations reported for porcine gelatin or agarose. The PVA-FX gels can be stored for long periods of time before exposure to radiation, since the auto-oxidation rate was 10 times less than that of gelatin-Fricke recipes. The new gels developed in this work are a significant improvement on previous Fricke gel systems.

Measurement of Gd-DTPA diffusion through PVA hydrogel using a novel magnetic resonance imaging method

Polyvinyl alcohol-cryogel (PVA-C) is a hydrogel that is an excellent tissue mimic. In order to characterize mass transfer in this material, as well as to demonstrate in principle the ability to noninvasively measure solute diffusion in tissue, we measured the diffusion coefficient of the magnetic resonance (MR) contrast agent gadolinium diethylene triaminopentaacetic acid (Gd-DTPA) through PVA-C using a clinical MR imager. The method involved filling thick-walled rectangular PVA-C "cups" with known concentrations of Gd-DTPA solutions. Then by using a fast inversion recovery spin echo MR imaging protocol, a signal "null" contour was created in the MR image that corresponded to a second, known concentration of Gd-DTPA. By collecting a series of MR images through the PVA-C wall as a function of time, the displacement of this second known isoconcentration contour could be tracked. Application of Fick's second law of diffusion yielded the diffusion coefficient. Seven separate experiments were performed using various combinations of initial concentrations of Gd-DTPA within the PVA-C cups (3.2, 25.6, or 125 mM) and tracked isoconcentrations contours (0.096, 0.182, or 0.435 mM Gd-DTPA). The experimental results and the predictions of Fick's law were in excellent agreement. The diffusivity of Gd-DTPA through 10% PVA hydrogel was found to be (2.6 +/- 0.04) x 10(-10) m(2)/s (mean +/- s.e.m.). Separate permeability studies showed that the diffusion coefficient of Gd-DTPA through this hydrogel did not change with an applied pressure of up to 7.1 kPa. Accurate measurements could be made within 30 min if suitable Gd-DTPA concentrations were selected. Due to the excellent repeatability and fast data acquisition time, this technique is very promising for future in vivo studies of species transport in tissue.

Lanthanide-EDTA doped agarose gels for use in NMR imaging phantoms

Gadolinium-ethylene-diamine-tetraacetic acid (Gd-EDTA) doped agarose gels have been used extensively to produce magnetic resonance imaging test materials. These materials are limited in their use in imagers operating at different frequencies and temperatures. The aim of this paper is to show that the theories used to predict the relaxation times for Gd-EDTA doped gels could be used for other paramagnetic lanthanide-EDTA gels with the aim of producing materials whose relaxation times can be theoretically determined but are less temperature and frequency dependent. In the present work, various lanthanide-EDTA gels were studied regarding their NMR T1 and T2 water proton relaxation time dependencies with temperature and proton Larmor frequency. Gd-EDTA doped agarose gels have a T1 relaxation time dependence with temperature of 7.5 ms/K. However an Er-EDTA doped gel has a temperature dependence of only 3.2 ms/K. Similarly Gd-EDTA doped agarose gels vary by 3.48 ms/MHz over the frequency range 2.5 MHz to 80 MHz compared to Ho-EDTA, which varies 0.359 ms/MHz over the same range. These results show that it is possible to produce lanthanide-EDTA doped agarose gels that have markedly reduced temperature and frequency dependences of their relaxation times, but are still predictable using existing theories. This would allow the direct comparison of the performance of imagers operating at different frequencies and temperatures.

Polyvinyl alcohol cryogel: an ideal phantom material for MR studies of arterial flow and elasticity

The authors present a unique application of polyvinyl alcohol (PVA) cryogel as an anthropomorphic, elastic, vascular phantom material that can be used in MR imaging. The composition consists of two nontoxic ingredients: water and PVA. The biomechanical and MR properties can be adjusted to be similar to those of excised porcine aortas by varying the number of freeze-thaw cycles to which the PVA solution is exposed. The authors present the T1, T2, shrinkage, and tensile properties of PVA cryogel tubes as a function of freeze-thaw cycles. MR images of a dual elastic aortic phantom undergoing pulsatile motion are shown.

Quantitative analyses of correlations of signal intensity on T1-weighted images and T1 relaxation time with copper concentration in the rat liver

RATIONALE AND OBJECTIVES

To evaluate the effects of intrahepatic copper on magnetic resonance (MR) images, we studied the signal intensity (SI) of T1-weighted images and the T1 relaxation time of Long-Evans Cinnamon (LEC) rats, which have abnormal copper metabolism, and compared them with those of Sprague-Dawley (SD) rats.

METHODS

We imaged the livers of four LEC rats before they developed hepatitis and four SD rats. The SI ratio of the liver to a phantom of polyvinyl alcohol gel was measured on T1-weighted images, and the T1 relaxation time was obtained from calculated T1 images. Copper concentration was measured by atomic absorption spectrophotometry.

RESULTS

The mean copper concentrations in the liver of LEC rats were approximately 20-fold and statistically higher than in SD rats. There was no significant difference in the SI ratio and the T1 relaxation time between the LEC and SD rats.

CONCLUSION

Intrahepatic copper does not significantly influence either the SI of T1-weighted MR images or the T1 relaxation time of the rat liver.

Non-invasive magnetic resonance imaging of the soft tissue response to a biomaterial

Magnetic resonance imaging (MRI) has been employed to visualize the tissue response to hydrogel implants in rats. High contrast MR images of the implant site were obtained. Distinct tissue variations in the MR images have been observed. These can be attributed to either the surgical procedure or the application of a tissue irritant to produce inflammation and have been verified histologically. This study demonstrates that MRI is potentially a useful tool for the non-invasive in-vivo evaluation of biomaterials.

An MRI phantom material for quantitative relaxometry

Most phantom media in current use exhibit T1 relaxation times that are significantly dependent on both temperature and operating frequency. This can introduce undesirable variability into relaxation measurements due to temperature fluctuations, and complicates direct comparison of imagers operating at different magnetic field strengths. Our investigations of a nickel-doped agarose gel system have demonstrated near independence of the proton relaxation rates to a wide range of temperatures and frequencies. We therefore propose the adoption of Ni2+ as a relaxation modifier for phantom materials used as relaxometry standards.