Radiation dosimetry in lung equivalent regions by use of polymer gel foams and quantitative magnetization transfer imaging

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How to scan polymer gels with MRI?

The absorbed radiation dose fixated in a polymer gel dosimeter can be read out by several methods such as magnetic resonance imaging (MRI), optical CT, X-ray CT and ultrasound with MRI being the first method that was explored. Although MRI was considered as an elegant scanning technique, readily available in most hospitals, it was later found that using a non-optimized imaging protocol may result in unacceptable deviations in the obtained dose distribution. Although most medical physicists have an understanding of the basic principles of magnetic resonance imaging (MRI), the optimization of quantitative imaging sequences and protocols is often perceived as the work of MRI experts. In this paper, we aim at providing the reader with some easy guidelines in how to obtain reliable quantitative MRI maps

Design of Anisotropic Diffusion Hardware Fiber Phantoms

A gold standard for the validation of diffusion weighted magnetic resonance imaging (DW-MRI) in brain white matter (WM) is essential for clinical purposes but still not available. Synthetic anisotropic fiber bundles are proposed as phantoms for the validation of DW-MRI because of their well-known structure, their long preservability and the possibility to create complex geometries such as curved and fiber crossings. A crucial question is how the different material properties and size of the fiber phantoms influence the outcome of the DW-MRI experiment. Several fiber materials are compared in this study. The effect of surface relaxation and internal gradients on the SNR is evaluated. In addition, the dependency of the fiber density and fiber radius on the diffusion properties is investigated

Radiation dosimetry by use of radiosensitive hydrogels and polymers : mechanisms, state-of-the-art and perspective from 3D to 4D

Gel dosimetry was developed in the 1990s in response to a growing need for methods to validate the radiation dose distribution delivered to cancer patients receiving high-precision radiotherapy. Three different classes of gel dosimeters were developed and extensively studied. The first class of gel dosimeters is the Fricke gel dosimeters, which consist of a hydrogel with dissolved ferrous ions that oxidize upon exposure to ionizing radiation. The oxidation results in a change in the nuclear magnetic resonance (NMR) relaxation, which makes it possible to read out Fricke gel dosimeters by use of quantitative magnetic resonance imaging (MRI). The radiation-induced oxidation in Fricke gel dosimeters can also be visualized by adding an indicator such as xylenol orange. The second class of gel dosimeters is the radiochromic gel dosimeters, which also exhibit a color change upon irradiation but do not use a metal ion. These radiochromic gel dosimeters do not demonstrate a significant radiation-induced change in NMR properties. The third class is the polymer gel dosimeters, which contain vinyl monomers that polymerize upon irradiation. Polymer gel dosimeters are predominantly read out by quantitative MRI or X-ray CT. The accuracy of the dosimeters depends on both the physico-chemical properties of the gel dosimeters and on the readout technique. Many different gel formulations have been proposed and discussed in the scientific literature in the last three decades, and scanning methods have been optimized to achieve an acceptable accuracy for clinical dosimetry. More recently, with the introduction of the MR-Linac, which combines an MRI-scanner and a clinical linear accelerator in one, it was shown possible to acquire dose maps during radiation, but new challenges arise

Uncertainty in 3D gel dosimetry

Three-dimensional (3D) gel dosimetry has a unique role to play in safeguarding conformal radiotherapy treatments as the technique can cover the full treatment chain and provides the radiation oncologist with the integrated dose distribution in 3D. It can also be applied to benchmark new treatment strategies such as image guided and tracking radiotherapy techniques. A major obstacle that has hindered the wider dissemination of gel dosimetry in radiotherapy centres is a lack of confidence in the reliability of the measured dose distribution. Uncertainties in 3D dosimeters are attributed to both dosimeter properties and scanning performance. In polymer gel dosimetry with MRI readout, discrepancies in dose response of large polymer gel dosimeters versus small calibration phantoms have been reported which can lead to significant inaccuracies in the dose maps. The sources of error in polymer gel dosimetry with MRI readout are well understood and it has been demonstrated that with a carefully designed scanning protocol, the overall uncertainty in absolute dose that can currently be obtained falls within 5% on an individual voxel basis, for a minimum voxel size of 5 mm(3). However, several research groups have chosen to use polymer gel dosimetry in a relative manner by normalizing the dose distribution towards an internal reference dose within the gel dosimeter phantom. 3D dosimetry with optical scanning has also been mostly applied in a relative way, although in principle absolute calibration is possible. As the optical absorption in 3D dosimeters is less dependent on temperature it can be expected that the achievable accuracy is higher with optical CT. The precision in optical scanning of 3D dosimeters depends to a large extend on the performance of the detector. 3D dosimetry with X-ray CT readout is a low contrast imaging modality for polymer gel dosimetry. Sources of error in x-ray CT polymer gel dosimetry (XCT) are currently under investigation and include inherent limitations in dosimeter homogeneity, imaging performance, and errors induced through post-acquisition processing. This overview highlights a number of aspects relating to uncertainties in polymer gel dosimetry

Verificatie van de dosisverdeling bij hogeprecisie-radiotherapie met behulp van NMR-geldosimetrie

Voor het verifieren van de dosisverdeling bij hogeprecisieradiotherapiebehandelingen in drie dimensies werden twee verschillende soorten gel gefabriceerd en onderzocht. Bij bestraling treedt in de Fricke-gel een oxidatie van ijzer(II)-ionen op tot vorming van ijzer(III)-ionen. Vermits beide ionen een verschillend magnetisch moment bezitten, kan de dosisverdeling uitgelezen worden bij middel van magnetische-resonantiebeeldvorming onder de vorm van R1-beelden. We hebben echter ondervonden dat deze methode onderhevig is aan spatiale instabiliteiten aangezien de ionen in staat zijn te diffunderen doorheen de gelmatrix. Omwille van die reden werd een tweede soort gel onderzocht. De monomeer-polymeergel daarentegen vertoont een betere ruimtelijke stabiliteit. Bij monomeer-polymeergeldosimetrie is de spinspinrelaxatiesnelheid, R2, gerelateerd aan de geabsorbeerde stralingsdosis in een gelfantoom. R2 wordt in elke pixel berekend door het fitten van een exponentiele vervalcurve met tijdsconstante 1/R2, aan de individuele pixelintensiteiten over de opeenvolgende basisbeelden. Verschillende beeldvormings-sequenties werden onderzocht en op punt gesteld voor het bekomen van de basisbeelden (i.e. enkelvoudige en meervoudige spin-echosequenties). De richtwaarde voor de nauwkeurigheid die aan gel-dosimetrie gesteld wordt, is 3% tot 5%. In een conventionele MR-scanner kunnen beeldvormingsartefacten voor aanzienlijke afwijkingen in de opgemeten dosisverdeling zorgen. Deze afwijkingen komen tot uiting onder de vorm van vervormingen van de dosisverdeling en onder de vorm van dosisafwijkingen. Deze artefacten worden veroorzaakt door verstoringen van het magnetisch veld en ruimtelijke variaties in het uitgezonden radiofrequente signaal. Anderzijds bevatten de basisbeelden tevens stochastische elektromagnetische ruis afkomstig van fantoom en omgeving. Bijgevolg zullen ook de dosisbeelden ruis bevatten. De signaal-ruisverhouding kan vergroot worden door een optimale keuze van scanparameters en door meerdere acquisities te nemen. Dit laatste gaat echter wel ten koste van meettijd. De kwantitatieve R2-sequentie werd geoptimaliseerd met betrekking tot de signaal-ruisverhouding en werd aangepast om systematische fouten te vermijden. Een idee van de globale nauwkeurigheid van de geldosimeter (bepaald door zowel chemische parameters als beeldvormingsparameters) werd bekomen door het vergelijken van verschillende opgemeten dosisverdelingen met deze bekomen bij middel van andere dosimetrietechnieken. In een eerste studie werden de diepte-dosiscurve en laterale profielen op een diepte van 5 cm en 10 cm opgenomen bij middel van zowel geldosimetrie als diamantdetector in een automatisch waterfantoom. In een tweede studie werd zowel de geldosimetrie als de 3D-dosisverificatiemethode voor conforme radiotherapie-behandelingen geevalueerd in een antropomorf fantoom. Hiertoe werden verschilbeelden berekend van dosisbeelden bekomen met geldosimetrie en bekomen met filmdosimetrie en computerplanning. In een afstandsgebaseerde vergelijking werd de afstand tussen de isodosisoppervlakken bepaald. Mits compensatie van allerhande beeldvormingsartefacten, komt geldosimetrie tegemoet aan de verwachtingen die aan deze nieuwe 3D-dosimetrietechniek gesteld worden. Toekomstig onderzoek is gericht naar een beter begrip van de fysico-chemische mechanismen, andere (minder-toxische) monomeren, andere scantechnieken en longequivalente gels

Evaporation and diffusion of chloroform with the deformable FlexyDos3D radiation dosimeter

Chloroform in the FlexyDos3D dosimeter acts as a radical initiator which brings about the colour change of the dosimeter when the radicals react with the leucomalachite green. However, the volatility of the chloroform likely results in a rapid loss of chloroform from the dosimeter. Gravimetric analysis and NMR diffusion-ordered coherence spectroscopy were used to measure the diffusion and evaporation rates of chloroform from the dosimeter and found that both rates were both significantly large resulting in a rapid loss of chloroform. Dose maps of irradiated phantoms aged for different times found a significant difference in the dose measurements of the dosimeter, likely a result of the different chloroform concentrations within the dosimeters

The effect of experimental low back pain on lumbar muscle activity in people with a history of clinical low back pain: a muscle functional MRI study

In people with a history of low back pain (LBP), structural and functional alterations have been observed at several peripheral and central levels of the sensorimotor pathway. These existing alterations might interact with the way the sensorimotor system responds to pain. We examined this assumption by evaluating the lumbar motor responses to experimental nociceptive input of 15 participants during remission of unilateral recurrent LBP. Quantitative T2 images (muscle functional MRI) were taken bilaterally of multifidus, erector spinae, and psoas at several segmental levels (L3 upper and L4 upper and lower endplate) and during several conditions: 1) at rest, 2) upon trunk-extension exercise without pain, and 3) upon trunk-extension exercise with experimental induced pain at the clinical pain-side (1.5-ml intramuscular hypertonic saline injections in erector spinae). Following experimental pain induction, muscle activity levels similarly reduced for all three muscles, on both painful and nonpainful sides, and at multiple segmental levels (P = 0.038). Pain intensity and localization from experimental LBP were similar as during recalled clinical LBP episodes. In conclusion, unilateral and unisegmental experimental LBP exerts a generalized and widespread decrease in lumbar muscle activity during remission of recurrent LBP. This muscle response is consistent with previous observed patterns in healthy people subjected to the same experimental pain paradigm. It is striking that similar inhibitory patterns in response to pain could be observed, despite the presence of preexisting alterations in the lumbar musculature during remission of recurrent LBP. These results suggest that motor output can modify along the course of recurrent LBP

Optimization for stability of the deformable FlexyDos3D radiation dosimeter and curing effects

Previous formulations of the FlexyDos3D dosimeter have shown significant changes in the dose-response over time. In this study, various formulations of the dosimeter were created and tested to see if this stability could be improved. A dosimeter that was stable over a three-day period was found. Rapid manufacture of this dosimeter for patient-specific validation of radiotherapy treatments is desirable. The use of 3D printing manufacturing techniques for thermosetting polymers require high temperatures to cure the polymer within a reasonable time. The effect of different curing temperatures and times were investigated for the FlexyDos3D radiation dosimeter for its effect on stability. No significant difference in the dose-response was found for dosimeters cured for different curing times beyond an hour. A significant dose-response offset was found between dosimeters cured at different temperatures, but the dose-response sensitivity was the same