Dosimetric characterization of CyberKnife radiosurgical photon beams using polymer gels

Dose distributions registered in water equivalent, polymer gel dosimeters were used to measure the output factors and off-axis profiles of the radiosurgical photon beams employed for CyberKnife radiosurgery. Corresponding measurements were also performed using a shielded silicon diode commonly employed for CyberKnife commissioning, the PinPoint ion chamber, and Gafchromic EBT films, for reasons of comparison. Polymer gel results of this work for the output factors of the 5, 7.5, and 10 mm diameter beams are (0.702 +/- 0.029), (0.872 +/- 0.039), and (0.929 +/- 0.041), respectively. Comparison of polymer gel and diode measurements shows that the latter overestimate output factors of the two small beams (5% for the 5 mm beam and 3% for the 7.5 mm beams). This is attributed to the nonwater equivalence of the high atomic number silicon material of the diode detector. On the other hand, the PinPoint chamber is found to underestimate output factors up to 10% for the 5 mm beam due to volume averaging effects. Polymer gel and EBT film output factor results are found in close agreement for all beam sizes, emphasizing the importance of water equivalence and fine detector sensitive volume for small field dosimetry. Relative off-axis profile results are in good agreement for all dosimeters used in this work, with noticeable differences observed only in the PinPoint estimate of the 80%-20% penumbra width, which is relatively overestimated.

Shear bond strength of three veneering resins to a Ni-Cr alloy using two bonding procedures

Composite veneering materials are used as alternatives to porcelain in fixed prosthodontics. Mechanical retention of the resin on the metal framework has been associated with the formation of gaps at the resin/alloy interface, and failure of the restoration. Several chemical bonding systems have been introduced to promote resin adhesion. The purpose of the present study was to evaluate the shear bond strength of three photocured composites (Artglass, Solidex & Signum+) to a Ni-Cr alloy.72 wax disks covered with 150-mum diameter beads were cast and divided in two equal groups. In the first group, Metal Photo Primer was applied on the casting surface, while the Siloc system was used in the second. Each group was divided in three subgroups of 12 samples, in which the three composites were photocured. Half of the specimens of each subgroup were subjected to 1000 and 5000 thermal cycles (5 and 55 degrees C) respectively. All specimens were tested in shear in a universal testing machine. The Siloc-Solidex group showed the highest bond strength (17.3 +/- 3.7 MPa). No statistically significant difference was found between specimens treated with Metal Photo Primer or Siloc. Thermocycling did not significantly affect the bond strength values. Solidex showed an adhesive failure mode for both alloy surface treatments, while Artglass and Signum+ exhibited combination failures. Conclusively, the appropriate alloy surface treatment - resin combination can significantly improve the resin-alloy shear bond strength. Specifically, Solidex resin exhibited significantly higher bond values compared with Artglass and Signum+, for both surface treatments and thermocycling procedures.

Magnetic resonance angiography, function and viability evaluation in patients with Kawasaki disease

OBJECTIVES

We evaluated the ability of magnetic resonance imaging to perform a noninvasive assessment of coronary arteries, function and viability in one examination in a population with Kawasaki disease.

BACKGROUND

Magnetic resonance angiography (MRA) can identify coronary abnormalities in patients with Kawasaki disease (KD). Contrast enhanced cardiovascular magnetic resonance (CeCMR) is the current gold standard for scar detection. Steady-state, free precession (SSFP) cine is a reliable technique to evaluate myocardial function and wall motion.

METHODS

Twenty patients with KD aged 7-12 yrs, were examined. Coronary MRA was performed using a 1.5 T system with two ECG-triggered pulse sequences. CeCMR images were acquired 15 minutes after the i.v. injection of 0.1 mmol/kg Gd-DTPA using an inversion recovery sequence. SSFP cines were acquired using 6-mm short-axis slices from the atrioventricular ring to the apex.

RESULTS

Aneurysms of the coronary arteries were identified in 7 patients and coronary ectasia was present in the remaining 12 patients while 1 patient had both. Transmural anterior-apical scar was detected by ceCMR in two cases, while small inferior necrosis was identified in another 2 cases. Left ventricular function was deteriorated only in the two patients with antero-apical infarction. The presence of myocardial infarction was detected in the territory supplied by the involved coronary artery.

CONCLUSION

In Kawasaki disease MRA, SSFP cine and ceCMR are able to perform noninvasive coronary artery evaluation, function and infarct detection in a single study.

R2 relaxometry with MRI for the quantification of tissue iron overload in beta-thalassemic patients

PURPOSE

To evaluate the usefulness of a time-efficient MRI method for the quantitative determination of tissue iron in the liver and heart of beta-thalassemic patients using spin-spin relaxation rate, R2, measurements.

MATERIALS AND METHODS

Images were obtained at 1.5 T from aqueous Gd-DTPA solutions (0.106-8 mM) and from the liver and heart of 46 beta-thalassemic patients and 10 controls. The imaging sequence used was a respiratory-triggered 16-echo Carr-Purcell-Meiboom-Gill (CPMG) spin-echo (SE) pulse sequence (TR = 2000 msec, TE(min) = 5 msec, echo spacing (ES) = 5 msec, matrix = 192 x 256, slice thickness = 10 mm). Liver iron concentration (LIC) measurements were obtained for 22 patients through biopsy specimens excised from the relevant liver segment. Biopsy specimens were also evaluated regarding iron grade and fibrosis. Serum ferritin (SF) measurements were obtained in all patients.

RESULTS

A statistically significant difference was found between patients and healthy controls in mean liver (P < 0.004) and myocardium (P < 0.004) R2 values. The R2 values correlated well with Gd DTPA concentration (r = 0.996, P < 0.0001) and LIC (r = 0.874, P < 0.0001). A less significant relationship (r = 0.791, P < 0.0001) was found between LIC measurements and SF levels. R2 measurements appear to be significantly affected (P = 0.04) by different degrees of hepatic fibrosis. The patients' liver R2 values did not correlate with myocardial R2 values (r = 0.038, P < 0.21).

CONCLUSION

Tissue iron deposition in beta-thalassemic patients may be adequately quantified using R2 measurements obtained with a 16-echo MRI sequence with short ES (5 msec), even in patients with a relatively increased iron burden.

Polymer gel dosimetry for the TG-43 dosimetric characterization of a new 125I interstitial brachytherapy seed

In this work, a polymer gel-magnetic resonance (MR) imaging method is employed for the dosimetric characterization of a new 125I low dose rate seed (IsoSeed model I25.S17). Two vials filled with PABIG gel were prepared in-house and one new seed as well as one commercially available 125I seed of similar dose rate and well-known dosimetric parameters (IsoSeed model I25.S06) were positioned in each vial. Both seeds in each vial were MR scanned simultaneously on days 11 and 26 after implantation. The data obtained from the known seed in each vial are used to calibrate the gel dose response which, for the prolonged irradiation duration necessitated by the investigated dose rates, depends on the overall irradiation time. Data for this study are presented according to the AAPM TG-43 dosimetric formalism. Polymer gel results concerning the new seed are compared to corresponding, published dosimetric results obtained, for the purpose of the new seed clinical implementation, by our group using the established methods of Monte Carlo (MC) simulation and thermo-luminescence dosimetry (TLD). Polymer gel dosimetry yields an average dose rate constant value of lambda = (0.921 +/- 0.031) cGy h(-1) U(-1) relative to (MC)lambda = (0.929 +/- 0.014) cGy h(-1) U(-1), (TLD)lambda = (0.951 +/- 0.044) cGy h(-1) U(-1) and the average value of Lambda = (0.940 +/- 0.051) cGy h(-1) U(-1) proposed for the clinical implementation of the new seed. Results for radial dose function, g(L)(r), and anisotropy function, F(r, theta), also agree with corresponding MC calculations within experimental uncertainties which are smaller for the polymer gel method compared to TLD. It is concluded that the proposed polymer gel-magnetic resonance imaging methodology could be used at least as a supplement to the established techniques for the dosimetric characterization of new low energy and low dose rate interstitial brachytherapy seeds.

An evaluation of the TSE MR sequence for time efficient data acquisition in polymer gel dosimetry of applications involving high doses and steep dose gradients

The use of magnetic resonance imaging as a readout method for polymer gel dosimetry commonly involves long imaging sessions, particularly when high spatial resolution is required in all three dimensions, for the investigation of dose distributions with steep dose gradients and stringent dose delivery specifications. In this work, a volume selective turbo spin echo (TSE) pulse sequence is compared to the established Carr-Purcell-Meiboom-Gill (CPMG) multiecho acquisition with regard to providing accurate dosimetric results in significantly reduced imaging times. Polyethylene glycol diacrylate based (PABIG) gels were irradiated and subsequently scanned to obtain R2 relaxation rate measurements, using a CPMG multiecho sequence and a dual echo TSE utilizing an acceleration (turbo) factor of 64. R2 values, plotted against corresponding Monte Carlo dose calculations, provided calibration data of PABIG gels dose response over a wide dose range. A linear R2 versus dose relationship was demonstrated for both sequences with TSE results presenting reduced dose sensitivity. Although TSE data were found to deviate from linearity at lower doses compared to CPMG data, a relatively wide dynamic dose range of response extending up to approximately 100 Gy was observed for both sequences. The TSE and CPMG sequences were evaluated with a brachytherapy irradiation using a high dose rate 192Ir source and a gamma knife stereotactic radiosurgery irradiation with a single 4 mm collimator helmet shot. Dosimetric results obtained with the TSE and CPMG are shown to compare equally well with the expected dose distributions for these irradiations. The 60-fold scan time reduction achieved with TSE implies that this sequence could prove to be a useful tool for the introduction of polymer gel dosimetry in clinical radiation therapy applications involving high doses and steep dose gradients.

Polymer gel dosimetry close to an125I interstitial brachytherapy seed

Despite its advantages, the polymer gel-magnetic resonance imaging (MRI) method has not, as yet, been successfully employed in dosimetry of low energy/low dose rate photon-emitting brachytherapy sources such as 125I or 103Pd interstitial seeds. In the present work, two commercially available 125I seed sources, each of approximately 0.5 U, were positioned at two different locations of a polymer gel filled vial. The gel vial was MR scanned with the sources in place 19 and 36 days after seed implantation. Calibration curves were acquired from the coupling of MRI measurements with accurate Monte Carlo dose calculations obtained simulating the exact experimental setup geometry and materials. The obtained gel response data imply that while linearity of response is sustained, sensitivity (calibration curve slope) is significantly increased (approximately 60%) compared to its typical value for the 192Ir (or 60Co and 6 MV LINAC) photon energies. Water equivalence and relative energy response corrections of the gel cannot account for more than 3-4% of this increase, which, therefore, has to be mainly attributed to physicochemical processes related to the low dose rate of the sources and the associated prolonged irradiation time. The calibration data obtained from one 125I source were used to provide absolute dosimetry results for the other 125I source, which were found to agree with corresponding Monte Carlo calculations within experimental uncertainties. It is therefore suggested that, regardless of the underlying factors accounting for the gel dose response to 125I irradiations, polymer gel dosimetry of new 125I or 103Pd sources should be carried out as originally proposed by Heard and Ibbot (2004 J. Phys.: Conf. Ser. 3 221-3), i.e., by irradiating the same gel sample with the new low dose rate source, as well as with a well-characterized low dose rate source which will provide the dose calibration curve for the same irradiation conditions.

Coronary Flow Evaluation by TIMI Frame Count and Magnetic Resonance Flow Velocity in Patients with Coronary Artery Ectasia

PURPOSE

Coronary artery ectasia (CAE) is defined as a dilatation of an arterial segment to a diameter at least 1.5 times that of the adjacent artery. It is characterized by slow flow, predisposing to thrombosis. The TIMI frame count (TFC) technique has been successfully used for the assessment of coronary flow velocity using coronary angiograms (CA). Our aim was to compare TFC with magnetic resonance peak flow velocity (PFV) for the evaluation of the coronary flow in patients with CAE.

METHODS

Fifteen male patients with CAE, aged 45-60 yrs, and 15 age-matched male controls were studied by both techniques. Only patients without coronary obstructive disease were included. Magnetic resonance coronary angiography (MRA) was performed with a 1.5T scanner. The most ectatic part of the proximal 1/3 of the vessel involved was examined. Velocity-encoded MR images were acquired in a double oblique imaging plane, which was perpendicular to the same ectatic segment. The findings were compared with TFC results.

RESULTS

Ten patients had RCA ectasia and five LAD ectasia. In CAE patients, peak flow velocity was 10.7 +/- 1.4 cm/sec in RCA and 11.4 +/- 2.3 cm/sec in LAD. TFC was 42.9 +/- 7.4 in RCA and 52.0 +/- 20.1 in LAD. In controls, peak flow velocity was 18.1 +/- 1.9 in RCA and 21.0 +/- 1.7 cm/sec in LAD. TFC was 20.4 +/- 1.59 in RCA and 19.8 +/- 1.12 in LAD. Controls had significantly higher peak flow velocity (p < 0.001) and lower TFC (p < 0.001) in both RCA and LAD, compared to patients with CAE. In patients with CAE, there was a negative correlation between PFV and TFC measurements (r = -0.74, p < 0.001).

CONCLUSION

Coronary flow in CAE patients can be assessed both by TFC and PFV. The noninvasive nature of PFV gives the opportunity for serial, easily repeatable, flow evaluation in these patients.

Three-dimensional dose verification of the clinical application of gamma knife stereotactic radiosurgery using polymer gel and MRI

This work seeks to verify multi-shot clinical applications of stereotactic radiosurgery with a Leksell Gamma Knife model C unit employing a polymer gel-MRI based experimental procedure, which has already been shown to be capable of verifying the precision and accuracy of dose delivery in single-shot gamma knife applications. The treatment plan studied in the present work resembles a clinical treatment case of pituitary adenoma using four 8 mm and one 14 mm collimator helmet shots to deliver a prescription dose of 15 Gy to the 50% isodose line (30 Gy maximum dose). For the experimental dose verification of the treatment plan, the same criteria as those used in the clinical treatment planning evaluation were employed. These included comparison of measured and GammaPlan calculated data, in terms of percentage isodose contours on axial, coronal and sagittal planes, as well as 3D plan evaluation criteria such as dose-volume histograms for the target volume, target coverage and conformity indices. Measured percentage isodose contours compared favourably with calculated ones despite individual point fluctuations at low dose contours (e.g., 20%) mainly due to the effect of T2 measurement uncertainty on dose resolution. Dose-volume histogram data were also found in a good agreement while the experimental results for the percentage target coverage and conformity index were 94% and 1.17 relative to corresponding GammaPlan calculations of 96% and 1.12, respectively. Overall, polymer gel results verified the planned dose distribution within experimental uncertainties and uncertainty related to the digitization process of selected GammaPlan output data.

Relationship of Number of Phases per Cardiac Cycle and Accuracy of Measurement of Left Ventricular Volumes, Ejection Fraction, and Mass

In cine cardiac magnetic resonance imaging (MRI) studies, for any preset imaging parameters the number of phases per cardiac cycle for a single slice is proportional to breath-hold duration. We investigated the relationship between the accuracy of measurement of left ventricular (LV) end-diastolic and end-systolic volumes (EDV and ESV, respectively), mass and ejection fraction (EF), and the number of phases acquired per cardiac cycle. Twelve adult volunteers underwent cardiac MRI and five complete LV functional studies were obtained with 8, 11, 14, 17, and 20 phases per cardiac cycle. We calculated LV volumes, EF, and mass for each acquisition, and compared them using the 20-phase acquisition as the reference standard. The scan duration was proportional to the number of phases acquired. There was a systematic underestimation of LV, EDV, and EF, with decreasing number of phases. Differences from the reference standard became significant for the 8-phase acquisition (p<0.05). Subgroup analysis showed that only those with slower heart rates (<65/min) had significant differences in EDV, but not in EF, for the 8-phase acquisition. For those with faster heart rates, no differences were detected between the different acquisitions. There were no significant differences between all acquisitions for the LV ESV and mass. We conclude that at least 11 phases per cardiac cycle are needed to maintain accuracy for cine cardiac MRI studies. Decreasing the number of phases per cardiac cycle beyond this cutoff may introduce significant error of measurement, particularly for the left ventricular EDV and EF and especially for those with bradycardia, and should be avoided.

Dose verification in clinical imrt prostate incidents

PURPOSE

In view of the need for dose-validation procedures on each individual intensity-modulated radiation therapy (IMRT) plan, dose-verification measurements by film, by ionization chamber, and by polymer gel-MRI dosimetry were performed for a prostate-treatment plan configuration. Treatment planning system (TPS) calculations were evaluated against dose measurements.

METHODS AND MATERIALS

Intensity-modulated radiation therapy (IMRT) treatments were planned on a commercial TPS. Kodak EDR-2 films were used for the verification of two-dimensional (2D) dose distributions at 1 coronal and 5 axial planes in a water-equivalent phantom. Full three-dimensional (3D) dose distributions were measured by use of a novel polymer gel formulation and a 3D magnetic resonance imaging (MRI) readout technique. Calculations were compared against measurements by means of isocontour maps, gamma-index maps (3% dose difference, 3-mm distance to agreement) and dose-volume histograms.

RESULTS

A good agreement was found between film measurements and TPS predictions for points within the 60% isocontour, for all the examined plans (gamma-index <1 for 96% of pixels). Three-dimensional dose distributions obtained with the polymer gel-MRI method were adequately matched with corresponding TPS calculations, for measurements in a gel phantom covering the planning-target volume (PTV).

CONCLUSIONS

Measured 2D and 3D dose distributions suggest that, for the investigated prostate IMRT plan configuration, TPS calculations provide clinically acceptable accuracy.

Correlation between magnetic resonance angiography (MRA) and quantitative coronary angiography (QCA) in ectatic coronary vessels

Coronary artery ectasia (CAE) is defined as a dilatation of an arterial segment to a diameter at least 1.5 times that of the adjacent normal artery. The correct follow-up of ectatic vessels is hampered by the need for repeat angiograms. In this work we compared quantitative coronary angiography (QCA) measurements of the diameter of the proximal most ectatic part of coronary vessels, with corresponding measurements obtained by magnetic resonance angiography (MRA) using both gradient echo and turbo spin echo imaging sequences. Fifteen patients (14 male), aged 45-65 years, with known CAE were prospectively studied. Two electrocardiogram (ECG)-triggered pulse sequences were implemented for coronary magnetic resonance angiography. The first was a three-dimensional (3D), segmented, k-space gradient-echo sequence, employing a T2-weighted preparation prepulse and a frequency-selective, fat-saturation prepulse to enhance "white blood" (WB) contrast of the coronary arteries. The second sequence was an M2D dual Inversion Recovery (IR) Turbo Spin-Echo with a linear k-space acquisition scheme, providing "black-blood" (BB) contrast of the coronaries. All scans were carried out with the patient free breathing using a 2D, real-time Navigator beam, for respiratory motion tracking and gating. All patients underwent QCA, and the diameter of the proximal most ectatic part of each vessel was measured and compared with "white-blood" and "black-blood" MRA measurements. The average length of continuously visualized LM, LAD, LCx, and RCA by MRA was 2.5 +/- 0.3, 5.8 +/- 0.8, 3.9 +/- 1.0, and 7.2 +/- 1.2 cm, respectively. There were no statistically significant differences between diameter measurements of the proximal most ectatic part of each vessel, obtained with WB and BB sequences. There was a close correlation between MRA and QCA measurements (r = 0.87, p < 0.001). Bland-Altman analysis showed no systematic differences between the examined methods, over the whole range of vessel diameters measured. Coronary MRA is in close correlation with QCA for CAE detection. Magnetic resonance angiography, being noninvasive, may prove of significant value for the efficient follow-up of these patients.

Magnetic resonance angiography is equivalent to X-ray coronary angiography for the evaluation of coronary arteries in Kawasaki disease

OBJECTIVES

The purpose of this study was to compare the results of magnetic resonance angiography (MRA) with X-ray coronary angiography (XCA) in a pediatric population.

BACKGROUND

Coronary artery abnormalities in Kawasaki disease (KD) develop in about 15% to 25% of young patients, mostly in the form of aneurysms.

METHODS

Thirteen patients (12 male), age three to eight years, were studied. The maximal diameter and length of the aneurysm were recorded. Coronary MRA was performed using a 1.5 T Philips Intera CV magnetic resonance scanner with an electrocardiographically triggered pulse sequence. It was a three-dimensional segmented k-space gradient-echo sequence (TE = 2.1 ms, TR = 7.5 ms, flip angle = 30 degrees, slice thickness = 1.5 mm) employing a T2-weighted preparation pre-pulse and a frequency selective fat-saturation pre-pulse. Data acquisition was performed in mid-diastole. All scans were carried out with the patient free breathing using a two-dimensional real-time navigator beam. All patients underwent XCA within a week.

RESULTS

In six patients, aneurysms of the coronary arteries were identified, while coronary ectasia alone was present in the remaining seven patients. Magnetic resonance angiography and XCA diagnosis of coronary artery aneurysm agreed completely. Maximal aneurysm diameter and length and ectasia diameter by MRA and XCA were similar. No stenotic lesion was identified by either technique.

CONCLUSIONS

In conclusion, MRA is a reliable diagnostic tool, equivalent to XCA for coronary artery aneurysm identification in patients with KD. Magnetic resonance angiography may prove to be of great value for the serial non-invasive evaluation of these patients.

3D dose verification in 192Ir HDR prostate monotherapy using polymer gels and MRI

VIPAR polymer gels and 3D MRI techniques were evaluated for their ability to provide experimental verification of 3D dose distributions in a simulation of a 192Ir prostate monotherapy clinical application. A real clinical treatment plan was utilized, generated by post-irradiation, CT based calculations derived from Plato BPS and Swift treatment planning systems. The simulated treatment plan involved the use of 10 catheters and 39 source positions within a glass vessel of appropriate dimensions, homogeneously filled with the VIPAR gel. 3D high resolution MR scanning of the gel produced T2 relaxation time maps, from which 3D dose distributions were derived via an appropriate calibration procedure. Results were compared to corresponding dose distributions obtained from the Plato and Swift treatment planning systems. Quantitative comparison, on a point by point basis, was based on user adopted acceptance criteria of 5% dose-difference and 3 mm distance-to-agreement. Significant deviations between experimental and calculated dose distributions were found for doses lower than 50% due to the reduced dose resolution of the method in the low dose, low dose gradient region. Measurement errors were observed at 1.0-1.5 mm around each catheter due to MR imaging susceptibility artifacts. For most remaining points the acceptance criteria were fulfilled. Systematic offsets of the order of 1-2 mm, observed between measured and corresponding calculated isocontours at specific segments, are attributed to the 1 mm uncertainty in catheter reconstruction and 1 mm uncertainty in the alignment of the MR and CT imaging planes.

Dosimetric calculations and VIPAR polymer gel dosimetry close to the microSelectron HDR

In the present study, different dosimetric methods were investigated for their ability to predict the energy dose in the vicinity of the microSelectron HDR 192Ir brachytherapy source. The results of a time-efficient Sievert integral model of proven accuracy in the cm distance range from all 192Ir sources were benchmarked against accurate Monte Carlo derived dosimetric data in the close vicinity of the source. This comparison revealed that the Sievert model is capable of accurate dosimetry even in the mm distance range from the source. The dose rate distributions were compared with results obtained from different versions (v. 13.7 and 14.2.2) of the Plato BPS commercial treatment planning system, for an application following the Paris trial intravascular irradiation protocol. The results of brachytherapy planning system calculations were found reliable at radial distances of clinical relevance. Noticeable errors existed only in the extreme case of dose calculations at 2 mm from the source axis using Plato v. 13.7. Experimental dosimetric data for the intravascular application, as obtained through the VIPAR polymer gel-MRI method, were also evaluated for dose verification purposes. This method allowed with reasonable accuracy the verification of absolute dose distributions for peripheral vessel applications using 192Ir sources.

Evaluation of the performance of VIPAR polymer gels using a variety of x-ray and electron beams

The aim of this investigation was the evaluation of the usefulness of N-vinyl pyrrolidone argon (VIPAR) polymer gel dosimetry for relative dose measurements using the majority of types and energies of radiation beams used in clinical practice. For this reason, VIPAR polymer gels were irradiated with the following beams: 6 and 23 MV photons (maximum dose: 15 Gy) and 6, 9, 12, 15, 18 and 21 MeV electrons (90% dose: 15 Gy). Using 6 MV x-rays, a linear gel dose response was verified for doses up to 20 Gy. Assuming linearity of response for the rest of the photon and electron beams used in this study, percentage depth dose measurements were derived. For all beams used and the range of relative doses studied, a satisfying agreement was observed between percentage depth dose measurements performed using the VIPAR gel-MRI method and an ion chamber, validating the assumption that a linear gel dose response holds for all photon and electron beams studied. VIPAR gels, therefore, can be used for relative dose distribution measurements using photons or electrons of any typical energy used in external radiotherapy applications. It is also demonstrated that two-dimensional dose distribution measurements through an irradiated (9 MeV electrons, 3 cm x 3 cm cone) VIPAR gel volume can be easily obtained.

Polymer gel dosimetry using a three-dimensional MRI acquisition technique

In this work, three-dimensional (3-D) MRI techniques are employed in N-Vinylpyrrolidone-Argon-(VIPAR-) based polymer gel dosimetry. VIPAR gels were irradiated using a Nucletron microSelection 192Ir HDR brachytherapy remote afterloading system with single source dwell position and intravascular brachytherapy irradiation protocols. A single VIPAR gel and a single irradiation are adequate to obtain the full calibration curve needed. The 3-D dose distributions obtained with the 3-D MRI method were found to be in good agreement with the corresponding Monte Carlo calculations, for brachytherapy and intravascular irradiations. The method allows the reconstruction of isodose contours over any plane, with increased spatial resolution and accuracy following a single MR acquisition. VIPAR gel measurements using a 3-D MRI readout technique can be of particular use in the experimental dosimetry of brachytherapy sources, as well as for dose verification purposes when complex irradiation regimes and three-dimensional dose gradients are investigated.

Wide dynamic dose range of VIPAR polymer gel dosimetry

In this work the extent of the linear dose response and the dynamic dose range of N-vinylpyrrolidone-argon based (VIPAR) polymer gels were investigated. VIPAR gels were irradiated using a 6 MV linear accelerator up to 60 Gy and a Nucletron microSelectron 192Ir HDR brachytherapy source to much higher doses to cover a dose range of two orders of magnitude. They were then MR scanned at 1.5 T to obtain T2-maps. VIPAR gel measurements obtained from the two irradiation regimes were calibrated against ion chamber measurements and dose calculations derived using the AAPM TG-43 protocol respectively. A satisfying agreement between the calibration results derived using the 6 MV x-rays and the 192Ir source was found for doses up to 60 Gy, implying that the response of the VIPAR gels is independent of photon energy and dose rate. A linear R2 dose response up to approximately 40 Gy and a dynamic dose range up to at least approximately 250 Gy were observed. VIPAR gel dose measurements derived using the monoexponentially fitted brachytherapy calibration data were found to be quite accurate.

Dosimetry close to an 192Ir HDR source using N-vinylpyrrolidone based polymer gels and magnetic resonance imaging

In this work, the utilization of polymer gel-MRI dosimetry for measurements at distances relevant to clinical brachytherapy and intravascular applications [i.e., in the mm range, where steep three-dimensional (3-D) dose gradients exist] is investigated using N-vinylpyrrolidone-based gels. Transverse axis radial dose distributions, dose distributions parallel to the source axis, and 2-D dose distributions around the commonly used microSelectron 192Ir HDR source are measured for single source dwell position irradiations. Experimental results are found in good agreement with verified Monte Carlo calculations, even for distances less than 3 mm from the source. The effect of various MRI parameters, such as slice thickness, slice mispositioning, and in-plane resolution, on the accuracy of the method is also investigated. Possible limitations of the method are discussed, and its' overall potential in brachytherapy dosimetry is evaluated. Experimental 2-D dose distributions for an intravascular application following the Paris irradiation protocol are compared to corresponding commercial treatment planning system calculations. Results suggest that polymer gel-MRI dosimetry is capable of experimentally verifying dose distributions in relevant clinical intravascular applications.

Narrow stereotactic beam profile measurements using N-vinylpyrrolidone based polymer gels and magnetic resonance imaging

In this work, polymer gel-MRI dosimetry (using VIPAR gels), radiographic film and a PinPoint ion chamber were used for profile measurements of 6 MV x-ray stereotactic beams of 5 and 10 mm diameter. The VIPAR gel-MRI method exhibited a linear dose response up to 32 Gy. VIPAR gels were found to resolve the penumbra region quite accurately, provided that the in-plane image resolution of the related T2-map is adequate (< or = 0.53 mm). T2-map slice thickness had no significant effect on beam profile data. VIPAR measurements performed with a spatial resolution of 0.13 mm provided penumbra widths (80%-20% distance) of 1.34 and 1.70 mm for the 5 and 10 mm cones respectively. These widths were found to be significantly smaller than those obtained with the film (2.23 mm for the 5 mm cone, 2.45 mm for the 10 mm cone) and PinPoint (2.25 mm for the 5 mm cone, 2.52 mm for the 10 mm cone) methods. Regarding relative depth dose measurements, good correlation between VIPAR gel and PinPoint data was observed. In conclusion, polymer gel-MRI dosimetry can provide relatively accurate profile data for very small beams used in stereotactic radiosurgery since it can overcome, to some extent, the problems related to the finite size of conventional detectors.

Detection of nitrosyl-iron complexes by proton-electron-double-resonance imaging

The nitrogen monoxide radical (NO*) forms paramagnetic mono- and dinitrosyl-iron complexes in biologic tissues. To establish a noninvasive technique for in vivo NO* imaging, we evaluated the suitability of these complexes as magnetic resonance (MR) contrast agents, making use of the ability of the unpaired electrons of the complexes to enter into dynamic nuclear polarization with water protons and hence produce enhancement on images generated by the technique of proton-electron-double-resonance imaging (PEDRI). Phantom solutions of synthetic nitrosyl-iron complexes (NICs) altered the signal intensity of PEDRI images. The dinitrosyl-iron complex (DNIC) with serum albumin induced a significantly larger signal alteration than the mononitrosyl-iron complex (MNIC) with dithiocarbamate. Exposure of rat liver to sodium nitroprusside (SNP) by ex vivo and in situ perfusion induced a composite X-band electron spin resonance (ESR) spectrum of the isolated liver characteristic of a MNIC and DNIC. On storage of the tissue, the MNIC signal disappeared and the DNIC signal intensity increased. Correspondingly, in cross-sectional PEDRI images taken at room temperature, the SNP-exposed livers initially exhibited a weak signal that strongly increased with time. In conclusion, NICs can be detected using PEDRI and could be exploited for in vivo NO* imaging.

Chemically induced analgesic nephropathy in the rat monitored by proton-electron double-resonance imaging (PEDRI)

Proton-electron double-resonance imaging (PEDRI) was used to assess renal function by monitoring the flow of the exogenous nitroxide free radical proxyl carboxylic acid (PCA) through normal and injured kidneys in the living rat. Kidney damage was induced by treatment with 2-bromoethylamine (BEA), which provides a well established model for human analgesic nephropathy. PCA clearance rates for liver, abdominal blood vessels, and renal tissues were determined from serial PEDRI images of normal rats (n = 6) and rats treated with BEA (n = 21). Different groups of BEA-treated animals were imaged on day 4 (n = 6), day 6 (n = 6), and day 9 (n = 9) after treatment. In BEA-treated rats, there was an increase in PCA half-life in all tissues studied. This increase was greatest in the kidney tissues and the effect progressed with time after treatment. The effect is probably due to BEA-induced damage to the tubules in the renal cortex and may not be related to the primary lesions in the renal medulla.

Nitroxide free radical clearance in the live rat monitored by radio-frequency CW-EPR and PEDRI

The use of RF (100 to 300 MHz) PEDRI and CW-EPR techniques allows the in vivo study of large animals such as whole rats and rabbits. Recently a PEDRI instrument was modified to also allow CW-EPR spectroscopy with samples of similar size and under the same experimental conditions. In the present study, this CW-EPR and PEDRI apparatus was used to assess the feasibility of the detection of a pyrrolidine nitroxide free radical (2,2,5,5,-tetramethylpyrrolidine-1-oxyl-3-carboxylic acid, PCA) in the abdomen of rats. In particular, we have shown that after the PCA administration (4 mmol kg(-1) b.w.): (i) the PCA EPR linewidth does not show line broadening due to concentration effects; (ii) a similar PCA up-take phase is observed by EPR and PEDRI; and (iii) the PCA half-lives in the whole abdomen of rats measured with the CW-EPR (T1/2=26+/-4 min, mean+/-sd, n=10) and PEDRI (T1/2=29+/-4 min, mean+/-sd, n=4) techniques were not significantly different (p > 0.05). These results show, for the first time, that information about PCA pharmacokinetics obtained by CW-EPR is the same as that from PEDRI under the same experimental conditions.

The application of PEDRI to the study of free radicals in vivo

Proton-electron double-resonance imaging (PEDRI) has considerable value for study of the distribution and elimination pathways of nitroxide free radicals (NFRs). This has been illustrated by its use in studies of kidney function in the living rat in which the NFR proxyl carboxylic acid (PCA) has been employed as a 'tracer'. The technique, at its present stage of development, can demonstrate location of PCA in enough detail to observe the passage through kidney cortex and medulla differentially, and to see the NFR within the major abdominal blood vessels. These studies are helping towards an understanding of the metabolic fate of PCA, as well as providing information about kidney performance after challenge with a nephrotoxin. In addition, nitric oxide complexes, formed in vivo by providing rats with a nitrite-rich diet, have been observed ex vivo using PEDRI and field-cycled DNP.