[Investigation of Verification Methods of Field Matching Using Polymer Gel Dosimeter in Proton Therapy].

PURPOSE: In our proton beam therapy center, we use imaging plates (IP) for dose verification of field matching in irradiation (called patch-field technique). In this study, a polymer gel dosimeter, which can perform three- dimensional dose distribution measurement, was used as a new tool for the verification of the patch field irradiation method corresponding to the rectangular irradiation. METHOD: The results of measurements of the PAGAT gel dosimeter to irradiate to two rectangular fields called as patch irradiation fields, which were created using the treatment planning system, were evaluated and compared to the results of IP and plan in profiles near the boundary of two fields. RESULT: In the case of no gap between the two fields, the relative dose using the gel dosimeter was 10.1% higher compared to that measured with the IP in the midpoint of two fields. In case of overlap (called hot region), the result of gels was 6.3% higher than that of IP. In the case of space (called cold region), the result of gels was 14.9% higher than that of IP. The results of the difference between gel and plan in the midpoint of two fields were 14.2% (no gap), -5.0% (hot), and 10.5% (cold). CONCLUSION: We found that the gel dosimeter was a 3-D dosimetric tool and possibility method for dose verification of patch fields. In this study, the results were preliminary and included several error factors. In the future, it is necessary to develop a dosimeter with improved and more precise measurements.

Influence of magnesium chloride on the dose-response of polyacrylamide-type gel dosimeters.

We investigated the effect of magnesium chloride (MgCl2) on the nuclear magnetic resonance dose-response of polyacrylamide-type (PAGAT, NIPAM, and VIPET) gel dosimeters containing acrylamide, N-isopropylacrylamide, and N-vinylpyrrolidone as a monomer, respectively. The dose-transverse relaxation rates (1/T2 = R2) obtained from magnetic resonance imaging data revealed that a substantial increase in the dose-R2 response occurred as the concentration of MgCl2 in the gel dosimeters increased. The sensitivity of the PAGAT gel with 1.0 M MgCl2 was found to be approximately one order higher than that of the same gel without MgCl2. In addition, the water equivalences of the gels with MgCl2 were evaluated over a wide range of photon energies. The results indicated that MgCl2 acts as a powerful sensitizer to radiation-induced free-radical polymerization in polyacrylamide-type gel dosimeters, but does not interfere with the desirable properties of basic polyacrylamide-type gel dosimeters (i.e., the dose rate and dose integration).

Wide slab is useful for routine quality control of MRI slice thickness.

Although slice thickness accuracy is important for the performance of magnetic resonance imaging systems, long scan times are required to perform reliable measurements. Inclined slabs and wedges are conventionally used as test devices to obtain slice profiles. In this study, a novel dedicated device with a widened slab was created, and its efficacy was compared with that of a conventional wedge. The signal-to-noise ratio (SNR) of the profile and the coefficient of variation (CV) of the measured slice thickness were measured. Wide slab usage showed sufficient SNR by averaging multiple profile lines, even with single acquisition. Therefore, it is possible to substantially shorten the measurement time. When ≥ 20 lines were averaged, CV was < 1%. Furthermore, a 200-mm slab width enabled evaluation of the positional dependence of slice thickness in a single imaging. Thus, quality control of MRI slice thickness can be easily implemented with this device.

[Optical Computed Tomography for Polymer Gel Dosimetry].

This article describes an optical computed tomography (OCT) for polymer gel dosimetry, focusing on two systems recently constructed by us. The first OCT system, which is categorized as a first-generation system, comprised a single He-Ne laser, photodiode, and mechanical stages for moving and turning gel dosimeter. Projection data per angle are acquired from turned gel dosimeter. In this system, the reconstructed image is obtained using filtered back-projection (FBP) method from projection data. The second OCT system is a 2D-OCT scanner that utilizes a light panel and a camera detector. The dose-response relationship between the optical density and radiation dose reconstructed from the scanning images is comparable to that obtained using magnetic resonance imaging. Although there is much room for improvement in the image artifacts due to reflection and refraction of light and so on, OCT is expected as a modality for the future polymer gel dosimetry as the rapid, high-resolution, highly accurate evaluation tool.

L Band EPR Tooth Dosimetry for Heavy Ion Irradiation.

Electron Paramagnetic Resonance (EPR) tooth dosimetry is being developed as a device to rapidly assess large populations that were potentially exposed to radiation during a major radiation accident or terrorist event. While most exposures are likely to be due to fallout and therefore involve low linear energy transfer (LET) radiation, there is also a potential for exposures to high LET radiation, for which the effect on teeth has been less well characterized by EPR. Therefore, the aim of this paper is to acquire fundamental response curves for high LET radiation in tooth dosimetry using L band EPR. For this purpose, we exposed human teeth to high energy carbon ions using the heavy ion medical accelerator in Chiba at the National Institute of Radiological Sciences. The primary findings were that EPR signals for carbon ion irradiation were about one-tenth the amplitude of the response to the same dose of 150 kVp X-rays.

Note: Utilization of polymer gel as a bolus compensator and a dosimeter in the near-surface buildup region for breast-conserving therapy.

Tangential beam radiotherapy is routinely used for radiation therapy after breast conserving surgery. A tissue-equivalent bolus placed on the irradiated area shifts the depth of the dose distribution; this bolus provides uniform dose distribution to the breast. The gel bolus made by the BANG-Pro(®) polymer gel and in an oxygen non-transmission pack was applicable as a dosimeter to measure dose distribution in near-surface buildup region. We validated the use of the gel bolus to improve in the whole-breast/chest wall, including the near-surface buildup region.

[Investigation of polymer gel dosimetry for small circular irradiated fields].

Polymer gels can be used as tissue equivalent dosimeters, and polymer gel dosimetry can be employed without perturbation of the radiation field. In this study, polymer gel dosimetry was used for small circular irradiation fields 10-30 mm in diameter using a radiation planning system. The irradiated gels were compared with planned data for a 50% dose width of 6 Gy dose maximum, and for the dose difference between gels and planned data over an 80% dose maximum area. The present study investigated magnetic resonance imaging (MRI) conditions based on an optimal dose-R2 calibration curve. The average difference between the full width half maximum of the 50% dose width between gels and planned data was 11%. The average dose difference over 80% of the dose was 5.6%. Optimal dose-R2 calibration curves were acquired using images with echo times of 30 and 60 ms. For cases of larger thicknesses and an increasing number of averages, the coefficients of variance of the curves were smaller than under other conditions. Compared to other traditional dosimetric tools, polymer gels have the advantage of providing three-dimensional dosimetric data. An arbitrary profile from the gel's data can be compared with the profile of the planned data. In the future, new gel dosimeters will be needed that demonstrate improved dose evaluation under 1 Gy and stability in high dose areas.

Feasibility of MR perfusion-weighted imaging by use of a time-spatial labeling inversion pulse.

Perfusion-weighted imaging (PWI) by use of arterial spin labeling (ASL) has been introduced to the clinical setting. However, it is not widely available because it requires specialized pulse sequences. Imaging using a time-spatial labeling inversion pulse (time-SLIP), which is a magnetic resonance angiography (MRA) technique that is based on ASL, can be used in various situations. In this study, we examined the feasibility of time-SLIP PWI. Two types of time-SLIP sequences were evaluated: (1) a single inversion recovery (IR) pulse sequence, which is the same as that used in conventional time-SLIP MRA except for the timing of data acquisition, and (2) a dual IR pulse sequence, where a second, non-selective, IR pulse was added during the inflow time to suppress background signals. Subtraction processing is performed between the "on" and "off" settings of the first IR pulse (time-SLIP tag) to obtain PWI. The average signal intensity was measured in a uniform phantom as the residual of the background, and in five healthy subjects as the perfusion signal. The average signal-to-noise ratio (SNR) was also measured in the five subjects. All imaging was performed with a 1.5-T MR scanner. Images using the dual IR method showed lower background signals and higher perfusion signals compared with images using the single IR method. However, the SNR was lower in images with the dual IR method. These results demonstrate that a time-SLIP, which is an MRA method, can be used for obtaining cerebral PWI simply by adjusting the imaging parameters.

Feasibility study on using imaging plates to estimate thermal neutron fluence in neutron-gamma mixed fields.

In current radiotherapy, neutrons are produced in a photonuclear reaction when incident photon energy is higher than the threshold. In the present study, a method of discriminating the neutron component was investigated using an imaging plate (IP) in the neutron-gamma-ray mixed field. Two types of IP were used: a conventional IP for beta- and gamma rays, and an IP doped with Gd for detecting neutrons. IPs were irradiated in the mixed field, and the photo-stimulated luminescence (PSL) intensity of the thermal neutron component was discriminated using an expression proposed herein. The PSL intensity of the thermal neutron component was proportional to thermal neutron fluence. When additional irradiation of photons was added to constant neutron irradiation, the PSL intensity of the thermal neutron component was not affected. The uncertainty of PSL intensities was approximately 11.4 %. This method provides a simple and effective means of discriminating the neutron component in a mixed field.

Usefulness of quantitative proton MR spectroscopy in the differentiation of benign and malignant meningioma.

This study was aimed to explore the value of quantitative proton MR spectroscopy (1H-MRS) in the differentiation of benign and malignant meningioma. 23 cases, including 19 benign (grade I) and 4 malignant (grade II-III) meningiomas, underwent single voxel 1H-MRS (TR/TE = 2000 ms/68, 136, 272 ms). T2 relaxation time of tissue water and choline were estimated by an exponential decay model. Choline concentration was calculated using tissue water as the internal reference, and corrected according to intra-voxel cystic/necrotic parts. Tissue water T2 of benign and malignant meningiomas were (105 +/- 41) ms and (151 +/- 42) ms, respectively. The difference was statistically significant (P = 0.033). While Choline T2 of benign and malignant meningiomas were (242 +/- 73) ms and (316 +/- 102) ms respectively, the difference was not significant (P = 0.105). Choline concentration was (2.86 +/- 0.86) mmol/ kg wet weight in benign meningiomas and (3.53 +/- 0.60) mmol/kg wet weight in malignant ones; after correction they increased to (2.98 +/- 0.93)mmol/kg wet weight and (4.58 +/- 1.22) mmol/kg wet weight, respectively, and the difference was significant (P = 0.019). In conclusion, quantitative 1H-MRS is useful for the differentiation of benign and malignant meningioma by T2 relaxation time and absolute choline concentration.

Time spatial labeling inversion pulse cerebral MR angiography without subtraction by use of dual inversion recovery background suppression.

Time-spatial labeling inversion pulse (time-SLIP), which is a technique of nonenhanced magnetic resonance angiography (MRA) based on arterial spin labeling (ASL), is used in various situations. Although subtraction between the images obtained with and without ASL is usually employed in cerebral time-SLIP MRA, to reduce the imaging time, dual inversion recovery (IR) has been applied for suppression of the background signals in this study. Appropriate timings for the 1st IR, 2nd IR, and the interval of data acquisition were investigated using computer simulation and a phantom experiment. With a short interval of data acquisition, the visibility of the simulated vessel was inadequate because replacement of the suppressed flow was insufficient. With a long interval of data acquisition, the contrast between the vessel and background was reduced. The reasons for this appeared to be the following: the longitudinal magnetization of the replaced flow is reduced because of the prolonged 2nd inversion time, causing a mismatch of the null point between the calculated and the actual values to become prominent. As a result, 3-4 s seemed to be an appropriate interval for data acquisition. Sufficient angiographic information could be obtained by use of dual IR background suppression in a volunteer study. With this technique, cerebral time-SLIP MRA can be performed in half of the imaging time required with the conventional subtraction technique.

Absolute choline concentration measured by quantitative proton MR spectroscopy correlates with cell density in meningioma.

INTRODUCTION: This study was aimed to investigate the relationship between quantitative proton magnetic resonance spectroscopy (1H-MRS) and pathological changes in meningioma. MATERIALS AND METHODS: Twenty-two meningioma cases underwent single voxel 1H-MRS (point-resolved spectroscopy sequence, repetition time/echo time = 2,000 ms/68, 136, 272 ms). Absolute choline (Cho) concentration was calculated using tissue water as the internal reference and corrected according to intra-voxel cystic/necrotic parts. Pathological specimens were stained with MIB-1 antibody to measure cell density and proliferation index. Correlation analysis was performed between absolute Cho concentration and cell density and MIB-1 labeled proliferation index. RESULTS: Average Cho concentration of all meningiomas before correction was 2.95 +/- 0.86 mmol/kg wet weight. It was increased to 3.23 +/- 1.15 mmol/kg wet weight after correction. Average cell density of all meningiomas was 333 +/- 119 cells/HPF, and average proliferation index was 2.93 +/- 5.72%. A linear, positive correlation between cell density and Cho concentration was observed (r = 0.650, P = 0.001). After correction of Cho concentration, the correlation became more significant (r = 0.737, P < 0.001). However, no significant correlation between Cho concentration and proliferation index was found. There seemed to be a positive correlation trend after correction of Cho concentration but did not reach significant level. CONCLUSION: Absolute Cho concentration, especially Cho concentration corrected according to intra-voxel cystic/necrotic parts, reflects cell density of meningioma.

New observations concerning the interpretation of magnetic resonance spectroscopy of meningioma.

This study was aimed to clarify some ambiguities in the interpretation of proton magnetic resonance spectroscopy (1H-MRS) of meningiomas. The cases of 31 meningioma patients (27 benign and 4 nonbenign meningiomas) that underwent single-voxel 1H-MRS (PRESS sequence, TR/TE = 2,000 ms/68, 136, 272 ms) were retrospectively analyzed. To verify the findings of in-vivo study, phantoms were measured, and pathological sections of 11 patients were reviewed. All meningiomas demonstrated increased choline and decreased creatine, except for a lipomatous meningioma that only displayed a prominent lipid (Lip) peak. Alanine (Ala) and lactate (Lac) coexisted in eight cases, indicating an alternative pathway of energy metabolism in meningiomas. They partially overlapped with each other and demonstrated a triplet-like spectral pattern, which was consistent with phantom study. Glutamine/glutamate (Glx) was helpful for the recognition of meningioma when Ala was absent. N-acetyl compounds(NACs) were observed in nine cases whose voxels were completely limited within the tumors, indicating that meningiomas might have endogenous NACs. Lac was indicative of an aggressive meningioma, although not always a nonbenign one. Lip not only represented micronecrosis in nonbenign meningiomas, but also reflected microcystic changes or fatty degeneration in benign meningiomas. 1H-MRS reflects some distinctive biochemical and pathological changes of meningiomas that might be misinterpreted.

Perfusion-weighted magnetic resonance imaging of the spinal cord in cervical spondylotic myelopathy.

Principles of echo shifting with a train of observations was used to perform magnetic susceptibility-weighted magnetic resonance imaging with bolus-tracking in 14 patients with spondylotic myelopathy to assess changes in perfusion parameters of the spinal cord before and after decompression surgery for cervical spondylotic myelopathy. The mean transit time (MTT), bolus arrival time (T0), and time to peak (TTP) were obtained from regions of interest (ROIs) and assessed as the ratio between the spinal cord and the pons (MTT index = MTT(ROI)/MTT(pons), T0 index = T0(ROI)/T0(pons), TTP index = TTP(ROI)/TTP(pons)). The patients were divided into two groups according to percentage improvement on the Neurosurgical Cervical Spine Scale. The MTT index in patients with good recovery (> or =50%) was significantly reduced. The T0 index and TTP index showed no significant change in both groups. Reduction of MTT index may indicate improved perfusion of the spinal cord following surgery for cervical spondylotic myelopathy.

Effect of J coupling and T2 Relaxation in Assessing of Methyl Lactate Signal using PRESS Sequence MR Spectroscopy.

PURPOSE: This work was aimed at quantification of lactate concentration using proton MR spectroscopy (MRS). We carried out a basic study to clarify the characteristics of signal change and T2 relaxation time of lactate that occur by J coupling in point resolved spectroscopy (PRESS) sequence. MATERIALS AND METHODS: Proton MRS was done for a water phantom containing 10 mmol/L creatine and lactate on a clinical 1.5 T MR system by using an asymmetric PRESS sequence. The coupling constant J was 7.35 Hz. In acquisitions, TE was varied from 68 ms up to 544 ms, with an increment of 68 ms (1/2J) and TR was fixed to 10000 ms. RESULTS: The shape and signal intensity of the lactate signal vary depending on its phase. The lactate signal intensity at TE 272 ms was higher than at TE 136 ms despite the longer TE. T2 relaxation times of lactate in the negative in-phase (TE 136 ms, TE 408 ms) and positive in-phase (TE 272 ms, TE 544 ms) were 1033 ms and 1042 ms, respectively (no significant differences), so that when the same phase was used, regardless of the phase condition, T2 relaxation behavior was not different. We considered that our results included over expression and loss of lactate signal depending on the phase. CONCLUSIONS: For evaluation of the lactate peak, we recommend the use of the positive in-phase signal because it is larger than the negative in-phase signal. The influence of the asymmetric PRESS sequence, which may cause loss and over expression of lactate signal, should be considered in the calculation of the quantification. The T2 relaxation time should be also considered in the calculation of the lactate value since it affects the value considerably.

Non-invasive quantification of lactate by proton MR spectroscopy and its clinical applications.

Lactate is an important metabolite in clinical cases indicating the status of metabolic impairment. We applied a clinically relevant simple method for lactate quantification using magnetic resonance spectroscopy (MRS). We used two long in-phase echo time (TE=272,544 ms) to calculate T2 relaxation time and the absolute concentration of lactate. This method was optimized using phantom study and applied to clinical cases. This technique does not require complicated processing and could be applied in daily clinical practice. Moreover, this technique enables lactate quantification in cases (e.g. tumor) where lipid peak is overlapped with the lactate peak at short echo times.

Correlation between choline and MIB-1 index in human gliomas. A quantitative in proton MR spectroscopy study.

BACKGROUND: Choline (Cho) containing compounds are usually evaluated using magnetic resonance spectroscopy (MRS) by relative ratios such as Cho/N-acetylaspartate (NAA) and Cho/creatine (Cre) ratios. To clarify the significance of Cho level in gliomas, we evaluated the quantified Cho level using MRS and compared it with the proliferation activity as determined by MIB-1 immunoreactivity in the histological specimen. METHODS: There were seven benign and seven malignant gliomas. MRS was obtained using a single-voxel proton regional imaging of metabolites (PRIME) sequence with three different TE for T2 compensation. Quantified Cho level was compared with the number of MIB-1 immunopositive positive cells and cell density in surgical specimens. RESULT: A positive correlation was observed between Cho and MIB-1 in benign gliomas, whereas there was a trend to an inverse correlation in malignant gliomas. This inverse correlation became a positive correlation when the necrotic area of the tumor (on the T1-weighted gadolinium enhanced images) was excluded from the voxel of interest (VOI) for MRS, but this correlation did not reach statistical significance. CONCLUSIONS: The quantification data clarified the behavior of Cho in malignant gliomas. The quantification method has the advantage of limiting the influence of other metabolites on Cho determination. In particular, the levels of other commonly measured metabolites, including Cre, may also be altered in glioma, making ratios between metabolites misleading. Heterogeneity in the MRS VOI should be considered when evaluating the proliferative activity of malignant glioma by MRS.