MRI diffusion phantoms: ADC and relaxometric measurement comparisons between polyacrylamide and agarose gels.

PURPOSE: The aim of this study is to compare polyacrylamide and agarose gels, as components of a simple MRI phantom, for the measurements of Apparent Diffusion Coefficient (ADC), T1 and T2 relaxation times. MATERIALS AND METHODS: Five (5) test tubes with polyacrylamide gels of different monomer concentrations and six (6) test tubes of different agarose gel concentrations were used as a phantom for ADC, T1 and T2 measurements, which were expressed as 2D color parametric maps, on a 1.5 T clinical MRI system. ADC and T2 maps were calculated utilizing a Weighted Linear (WL) regression fitting algorithm. T1 maps were calculated utilizing a standard non-linear fitting algorithm. RESULTS: In agarose gels, ADC measurements are independent of the agarose concentration, whereas the T1 and T2 relaxation times decrease with increasing agarose concentration. On the contrary, in polyacrylamide gels, ADC measurements decrease quadratically while increasing the monomer concentration, whereas the T1 and T2 relaxation times reveal a linear decrease with increasing monomer concentration. CONCLUSION: Polyacrylamide gels can serve as a better means for simulating ADC values, as compared with the agarose gels used in this study.

Dosimetric performance of the Elekta Unity MR-linac system: 2D and 3D dosimetry in anthropomorphic inhomogeneous geometry.

Following the clinical introduction of the Elekta Unity MR-linac, there is an urgent need for development of dosimetry protocols and tools, not affected by the presence of a magnetic field. This work presents a benchmarking methodology comprising 2D/3D passive dosimetry and involving on-couch adaptive treatment planning, a unique step in MR-linac workflows. Two identical commercially available 3D-printed head phantoms (featuring realistic bone anatomy and MR/CT contrast) were employed. One phantom incorporated a film dosimetry insert, while the second was filled with polymer gel. Gel dose-response characteristics were evaluated under the Unity irradiation and read-out conditions, using vials and a cubic container filled with gel from the same batch. Treatment plan for the head phantoms involved a hypothetical large C-shape brain lesion, partly surrounding the brainstem. An IMRT step-and-shoot 7-beam plan was employed. Pre-treatment on-couch MR-images were acquired in order for the treatment planning system to calculate the virtual couch shifts and perform adaptive planning. Absolute 2D and relative 3D measurements were compared against calculations related to both adapted and original plans. Real-time dose accumulation monitoring in the gel-filled phantom was also performed. Results from the vials and cubic container suggest that gel dose-response is linear in the dose range investigated and signal integrity is mature at the read-out timings considered. Head phantom 2D and 3D measurements agreed well with calculations with 3D gamma index passing rates above 90% in all cases, even with the most stringent criteria used (2 mm/2%). By exploiting the 3D information provided by the gel, comparison also involved DVHs, dose-volume and plan quality metrics, which also reflected the agreement between adapted and delivered plans within ±4%. No considerable discrepancies were detected between adapted and original plans. A novel methodology was developed and implemented, suitable for QA procedures in Unity. TPS calculations were validated within the experimental uncertainties involved.

Characterization of a novel 3D printed patient specific phantom for quality assurance in cranial stereotactic radiosurgery applications.

In single-isocenter stereotactic radiosurgery/radiotherapy (SRS/SRT) intracranial applications, multiple targets are being treated concurrently, often involving non-coplanar arcs, small photon beams and steep dose gradients. In search for more rigorous quality assurance protocols, this work presents and evaluates a novel methodology for patient-specific pre-treatment plan verification, utilizing 3D printing technology. In a patient's planning CT scan, the external contour and bone structures were segmented and 3D-printed using high-density bone-mimicking material. The resulting head phantom was filled with water while a film dosimetry insert was incorporated. Patient and phantom CT image series were fused and inspected for anatomical coherence. HUs and corresponding densities were compared in several anatomical regions within the head. Furthermore, the level of patient-to-phantom dosimetric equivalence was evaluated both computationally and experimentally. A single-isocenter multi-focal SRS treatment plan was prepared, while dose distributions were calculated on both CT image series, using identical calculation parameters. Phantom- and patient-derived dose distributions were compared in terms of isolines, DVHs, dose-volume metrics and 3D gamma index (GI) analysis. The phantom was treated as if the real patient and film measurements were compared against the patient-derived calculated dose distribution. Visual inspection of the fused CT images suggests excellent geometric similarity between phantom and patient, also confirmed using similarity indices. HUs and densities agreed within one standard deviation except for the skin (modeled as 'bone') and sinuses (water-filled). GI comparison between the calculated distributions resulted in passing rates better than 97% (1%/1 mm). DVHs and dose-volume metrics were also in satisfying agreement. In addition to serving as a feasibility proof-of-concept, experimental absolute film dosimetry verified the computational study results. GI passing rates were above 90%. Results of this work suggest that employing the presented methodology, patient-equivalent phantoms (except for the skin and sinuses areas) can be produced, enabling literally patient-specific pre-treatment plan verification in intracranial applications.

The impact of spin coupling signal loss on fat content characterization in multi-echo acquisitions with different echo spacing.

PURPOSE: This study aimed to assess the effect of echo spacing in transverse magnetization (T2) signal decay of gel and fat (oil) samples. Additionally, we assess the feasibility of using spin coupling as a determinant of fat content. METHODS: Phantoms of known T2 values, as well as vegetable oil phantoms, were scanned at 1.5T scanner with a multi echo FSE sequence of variable echo spacing above and below the empirical threshold of 20ms for echo train signal modulation (6.7, 13.6, 26.8, and 40ms). T2 values were calculated from monoexponential fitting of the data. Relative signal loss between the four acquisitions of different echo spacing was calculated. RESULTS: Agreement in the T2 values of water gel phantom was observed in all acquisitions as opposed to fat phantom (oil) samples. Relative differences in signal intensity between two successive sequences of different echo spacing on composite fat/water regions of interest was found to be linearly correlated to fat fraction of the ROI. CONCLUSION: The sample specific degree of signal loss that was observed between different fat samples (vegetable oils) can be attributed to the composition of each sample in J coupled fat components. Hence, spin coupling may be used as a determinant of fat content.

Occupational Electromagnetic Fields exposure in Magnetic Resonance Imaging systems - Preliminary results for the RF harmonic content.

PURPOSE: European legislation concerning the protection of workers from exposure to Electromagnetic Fields (EMF) was recently (26.6.2013) completed by Directive 2013/35/ΕU. This Directive is a specific one of the framework Directive 89/391/EEC and part of the overall legislation for Occupational Health and Safety (OHS). Magnetic Resonance Imaging (MRI) systems have played a key role, both in the postponement of the former 2004/40 EMF Directive and in the formation of the latest limits adopted by the new Directive. On the other hand, MRI systems are associated with the exposure of personnel to EMF of various frequencies and modulations, arousing peculiar safety issues. Therefore, we will try to acquire the highly important knowledge of the exact occupational exposure levels, in all working scenarios and practices. METHODS: Different MRI systems (1.5 and 3 T) have been chosen for a variety of measurements in order to assess occupational exposure compared to the limits (ALs) of the Directive and to the main OHS principles. Gradient function of MRI systems results in low frequency exposure, while high frequency exposure comes from the application of the RF excitation frequency. RESULTS: In most of the cases the RMS and peak value measurements do not exceed the corresponding ALs, apart from a few specific hot spots, manageable through OHS principles. DISCUSSION: Complete occupational exposure results can form the basis for dealing with multiple exposures present in MRI systems. Peculiar RF harmonic components, of no safety concern, were detected. Their origin is under examination.

Hemodynamic evidence linking cognitive deficits in clinically isolated syndrome to regional brain inflammation.

BACKGROUND AND PURPOSE: To investigate the relation between hemodynamic measurements and memory function in patients with clinically isolated syndrome (CIS). METHODS: Forty CIS patients were administered tests of verbal short-term/working memory and passage learning. Using dynamic susceptibility contrast MRI cerebral blood volume (CBV), cerebral blood flow and mean transit time values were estimated in 20 cerebral regions of interest, placed in normal appearing white matter (NAWM) and normal appearing deep gray matter structures, bilaterally. RESULTS: CIS patients showed significantly impaired scores on working memory and secondary verbal memory that correlated inversely with elevated CBV values in the left frontal and periventricular NAWM, thalamus, right caudate and corpus callosum. CONCLUSIONS: Verbal memory in CIS correlates inversely with elevated CBV values of brain structures involved in memory. As these hemodynamic changes, detected in CIS, are indicative of inflammation, the observed cognitive disturbances may relate to widespread brain inflammatory processes that prevail in early multiple sclerosis.

Technical note: a fast laser-based optical-CT scanner for three-dimensional radiation dosimetry.

PURPOSE: To introduce a novel laser-based optical-CT scanner for the readout of three-dimensional (3D) radiation dosimeters. METHODS: The scanner employs a diode laser, a cylindrical lens, a motorized linear rail, a rotation stage, and a charge-coupled device camera. The scanner operates in a translate-rotate fashion and may be set up in two configurations depending on the orientation of the cylindrical lens. The attenuation coefficient versus dose response was determined for a normoxic N-vinylpyrrolidone based polymer gel dosimeter. Cylindrical dosimeters, 2 cm diameter, were homogenously irradiated to known doses up to 60 Gy using a 6 MV linear accelerator. For a test irradiation, a 5 cm diameter dosimeter was irradiated along its cylindrical axis using a rectangular 1 cm x 1 cm irradiation beam. The dose readout of this scanner was compared to the corresponding readout of a common wide illumination and area detector optical-CT scanner. RESULTS: The attenuation coefficient versus dose response of the laser-based system was found to be linear up to 60 Gy (r2 = 0.997) compared to the wide field illumination based optical-CT scanner, which exhibits linearity up to 32 Gy (r2 = 0.996). The noise in the reconstructed attenuation coefficient maps was +/- 7.2 x 10(-2) mm(-1) versus +/- 9.5 x 10(-3) mm(-1) for the laser-based system and the wide field illumination system, respectively. CONCLUSIONS: We have developed a novel laser-based optical-CT scanner, which is capable of generating fast 3D dosimetric data using a scattering polymer gel dosimeter. Our data demonstrate that the dose readout of this scanner preserves the advantage of existing laser-based optical-CT scanners in providing measurements, which are minimally affected by scattered light. For accurate reconstruction of the attenuation coefficients, noise reduction techniques need to be applied.

Small SRS photon field profile dosimetry performed using a PinPoint air ion chamber, a diamond detector, a novel silicon-diode array (DOSI), and polymer gel dosimetry. Analysis and intercomparison.

Small photon fields are increasingly used in modern radiotherapy and especially in IMRT and SRS/SRT treatments. The uncertainties related to small field profile measurements can introduce significant systematic errors to the overall treatment process. These measurements are challenging mainly due to the absence of charged particle equilibrium conditions, detector size and composition effects, and positioning problems. In this work four different dosimetric methods have been used to measure the profiles of three small 6 MV circular fields having diameters of 7.5, 15.0, and 30.0 mm: a small sensitive volume air ion chamber, a diamond detector, a novel silicon-diode array (DOSI), and vinyl-pyrrolidone based polymer gel dosimeter. The results of this work support the validity of previous findings, suggesting that (a) air ion chambers are not suitable for small field dosimetry since they result in penumbra broadening and require significant corrections due to severe charged particle transport alterations; (b) diamond detectors provide high resolution and rather accurate small field profile measurements, as long as positioning problems can be addressed and the necessary dose rate corrections are correctly applied; and (c) the novel silicon-diode array (DOSI) used in this study seems to be adequate for small field profile measurements overcoming positioning problems. Polymer gel data were assumed as reference data to which the other measurement data were compared both qualitatively and quantitatively using the gamma-index concept. Polymer gels are both phantom and dosimeter, hence there are no beam perturbation effects. In addition, polymer gels are tissue equivalent and can provide high-spatial density and high-spatial resolution measurements without positioning problems, which makes them useful for small field dosimetry measurements. This work emphasizes the need to perform beam profile measurements of small fields (for acceptance, commissioning, treatment planning systems data feed, and periodic quality assurance purposes) using more than one dosimetric method. The authors believe this to be a safe way towards the reduction of the overall uncertainty related to SRS/SRT treatments.

An evaluation of the dosimetric performance characteristics of N-vinylpyrrolidone-based polymer gels.

The aim of this work was to investigate the dosimetric performance properties of the N-vinylpyrrolidone argon (VIPAR) based polymer gel as a dosimetric tool in clinical radiotherapy. VIPAR gels with a larger concentration of gelatin than the standard recipe were manufactured and irradiated up to 68 Gy using a 6 and 18 MV linear accelerator. Using MRI, the R2-dose response was recorded at different imaging sessions within a 34 day time period post-irradiation. The R2-dose response was found to be linear between 5 and 68 Gy. Although dose sensitivity did not show significant variation with time, the measured R2-dose values showed an increasing trend, which was less evident beyond 17 days. At one day post-irradiation, calculated dose standard uncertainties at 20 Gy and 56 Gy were 2.2% and 1.7%, providing a dose resolution of 0.45 Gy and 0.97 Gy, respectively. Although these values fulfilled the 2% limit of ICRU, when gels were imaged at one day post-irradiation, it was shown that the temporal evolution of the R2 values deteriorated the per cent standard uncertainty and the dose resolution by approximately 57%, when imaged 17 days post-irradiation. Variation in the coagulation temperature of the gels did not impact the R2-dose sensitivity. This study has shown that the VIPAR gel has the properties of a dosimetric tool required in clinical radiotherapy, especially in applications where a wide dose dynamic range is employed. For results with the lowest per cent uncertainty and the optimum dose resolution, the dosimetry gels used in this work should be MR scanned at one day post-irradiation. Furthermore, a preliminary study on the R2-dose response of a new normoxic N-vinylpyrrolidone-based polymer gel showed that it could potentially replace the traditional VIPAR gel formulation, while preserving the wide dynamic dose response inherent to that monomer.

Experimental determination of the effect of detector size on profile measurements in narrow photon beams.

The aim of this work is to investigate experimentally the detector size effect on narrow beam profile measurements. Polymer gel and magnetic resonance imaging dosimetry was used for this purpose. Profile measurements (Pm(s)) of a 5 mm diameter 6 MV stereotactic beam were performed using polymer gels. Eight measurements of the profile of this narrow beam were performed using correspondingly eight different detector sizes. This was achieved using high spatial resolution (0.25 mm) two-dimensional measurements and eight different signal integration volumes A X A X slice thickness, simulating detectors of different size. "A" ranged from 0.25 to 7.5 mm, representing the detector size. The gel-derived profiles exhibited increased penumbra width with increasing detector size, for sizes >0.5 mm. By extrapolating the gel-derived profiles to zero detector size, the true profile (Pt) of the studied beam was derived. The same polymer gel data were also used to simulate a small-volume ion chamber profile measurement of the same beam, in terms of volume averaging. The comparison between these results and actual corresponding small-volume chamber profile measurements performed in this study, reveal that the penumbra broadening caused by both volume averaging and electron transport alterations (present in actual ion chamber profile measurements) is a lot more intense than that resulted by volume averaging effects alone (present in gel-derived profiles simulating ion chamber profile measurements). Therefore, not only the detector size, but also its composition and tissue equivalency is proved to be an important factor for correct narrow beam profile measurements. Additionally, the convolution kernels related to each detector size and to the air ion chamber were calculated using the corresponding profile measurements (Pm(s)), the gel-derived true profile (Pt), and convolution theory. The response kernels of any desired detector can be derived, allowing the elimination of the errors associated with narrow beam profile measurements.

Relative output factor measurements of a 5 mm diameter radiosurgical photon beam using polymer gel dosimetry.

Besides the fine spatial resolution inherent in polymer gel-magnetic resonance imaging (MRI) dosimetry, the method also features the potential for multiple measurements of varying sensitive volume in a single experiment by integrating results in MRI voxels of finite dimensions (i.e., in plane resolution by slice thickness). This work exploits this feature of polymer gel dosimetry to propose an experimental technique for relative output factor (OF) measurements of small radiosurgical beams. Two gel vials were irradiated with a 5 and 30 mm diameter 6 MV radiosurgery beam and MR scanned with the same slice thickness and three different in plane resolutions. Using this experimental data set, 5 mm OF measurements with the PinPoint ion chamber are simulated by integrating results over a sensitive volume equal to that of the chamber. Results are found in agreement within experimental uncertainties with actual PinPoint measurements verifying the validity of the proposed experimental procedure. The polymer gel data set is subsequently utilized for OF measurements of the 5 mm beam with varying sensitive volume to discuss the magnitude of detector volume averaging effects. Seeking to correct for volume averaging, results are extrapolated to zero sensitive volume yielding a 5 mm OF measurement of (0.66+/-5%). This result compares reasonably with corresponding ionometric and radiographic film measurements of this work and corresponding, limited, data in the literature. Overall, results suggest that polymer gel dosimetry coupled with the proposed experimental procedure helps overcome not only tissue-equivalence and beam perturbation implications but also volume averaging and positioning uncertainties which constitute the main drawback in small radiosurgical beam dosimetry.

Detection of malignant bone marrow involvement with dynamic contrast-enhanced magnetic resonance imaging.

BACKGROUND: The purpose of this study was to evaluate the role of dynamic contrast-enhanced magnetic resonance imaging (dMRI) in detecting bone marrow involvement in cancer patients. PATIENTS AND METHODS: We studied 50 consecutive patients with histologically confirmed malignant dissemination to the bone marrow, using dMRI of the lumbosacral spine. Time-signal intensity curves were generated from regions of interest (ROIs) obtained from areas of obvious bone marrow disease (group B). In 16 patients from group B with focal disease, ROIs were also placed on areas with apparently normal bone marrow on static magnetic resonance images (group C). Twenty-two patients with no history of malignancy were used as a control group (group A). Wash-in (WIN) and wash-out (WOUT) rates, time to peak (TTPK), time to maximum slope (TMSP) values and WIN/TMSP ratios were calculated for each patient. Mean values for the three groups were compared statistically. Six patients from group B had follow-up dMRI after chemotherapy: four patients achieved a clinical partial response and two had resistant disease. RESULTS: A significant difference was found between groups A and B for all values. Between groups A and C, in spite of the similar static MRI appearance, all values were significantly different. Between groups B and C, a significant difference was found for WIN, WOUT rates and WIN/TMSP ratio. Follow-up dMRI data analysis correlated well with clinical staging. CONCLUSIONS: dMRI can distinguish normal from malignant bone marrow. It may identify malignant bone marrow infiltration in patients with negative static MRI and serve as both a diagnostic and prognostic tool for patients with bone marrow malignancies.

55Mn NMR investigation of electronic phase separation in La1-xCaxMnO3 for 0.2 < or = x < or = 0.5.

55Mn NMR line shape measurements in La1-xCaxMnO3 for 0.20< or =x< or =0.50 provide experimental evidence about the existence of two distinct regions in the T-x magnetic phase diagram, where the homogeneous ferromagnetic (FM) metallic state is separated into FM metallic and FM insulating regions. These results are in agreement with recent theoretical predictions, which reveal a novel electronic phase separation in two FM states, providing orbital ordering and Jahn-Teller phonons are taken into consideration.

Respiratory function in patients with thalassaemia and iron overload.

Iron deposition in the respiratory system has been proposed as a potential cause of the ventilatory restrictive impairment seen in patients with thalassaemia major (TM) and iron overload. In this study, magnetic resonance imaging (MRI) measurements of the liver (T2 relaxation time) were used as a surrogate index of total body iron burden and the extent to which these measurements correlated with total lung capacity (TLC) in patients with TM was examined. Twenty-one patients (aged 25+/-5 yrs) with TM participated in the study. Standard pulmonary function tests were undertaken and the T2 relaxation time of the liver was measured in all patients. Ventilatory restrictive impairment (mean TLC 74+/-11 (SD)% predicted) was the most common abnormality found in 71% of TM patients. There was no correlation between TLC (% pred) and T2 relaxation time (r=0.06, p=0.78). T2 relaxation time correlated weakly with average serum ferritin levels (r=-0.56, p=0.008). In conclusion, the data do not support the notion that the restrictive impairment in patients with thalassaemia major and iron overload is related to iron deposition in the respiratory system.

Assessment of liver iron overload by T2-quantitative magnetic resonance imaging: correlation of T2-QMRI measurements with serum ferritin concentration and histologic grading of siderosis.

PURPOSE: To correlate hepatic 1/T2 values obtained by means of a T2-Quantitative MRI (T2-QMRI) technique with three widely applied methods for the evaluation of hemosiderosis, i.e., (a) liver iron concentrations (LFeC) (b) serum ferritin (SF), and (c) histologic grading of siderosis. The impact of coexisting hepatitis was also considered. T2-QMRI measurements were compared with signal intensity (SI) ratio measurements on conventional SE images. MATERIALS AND METHODS: Liver T2 relaxation times were calculated in 40 thalassemic patients, on a 0.5 T magnetic resonance imaging system using a multiple spin-echo sequence with parameters: TR = 2500 ms, TE = 12 ms in 20 symmetrically repeatable echoes. RESULTS: (a) 1/T2 values were well correlated (r = 0.97) with liver iron concentrations, which ranged from 2.32 to 18.0 mg/g dry weight (normal < 1.6 mg/g). (b) 1/T2 values were also correlated with serum ferritin levels (r = 0.84). At various 1/T2 values, serum ferritin levels were higher for the anti-HCV(+) patients than the anti-HCV(-) ones. (c) T2 values corresponding to successive grades of siderosis presented statistically significant differences. (d) SI ratio measurement assigned less statistically significant results, as compared to T2 values. CONCLUSION: T2-QMRI measurement of T2 relaxation time is more accurate than SI ratios in evaluating liver iron overload. It is particularly useful for hemosiderotic patients with coexisting hepatitis since, in this case, serum ferritin is not considered a reliable index of hemosiderosis.