| 1. |
- Kuess, Peter, et al.
(author)
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Association between pathology and texture features of multi parametric MRI of the prostate
- 2017
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In: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 62:19, s. 7833-7854
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Journal article (peer-reviewed)abstract
- The role of multi-parametric (mp)MRI in the diagnosis and treatment of prostate cancer has increased considerably. An alternative to visual inspection of mpMRI is the evaluation using histogram-based (first order statistics) parameters and textural features (second order statistics). The aims of the present work were to investigate the relationship between benign and malignant sub-volumes of the prostate and textures obtained from mpMR images. The performance of tumor prediction was investigated based on the combination of histogram-based and textural parameters. Subsequently, the relative importance of mpMR images was assessed and the benefit of additional imaging analyzed. Finally, sub-structures based on the PI-RADS classification were investigated as potential regions to automatically detect maligned lesions. Twenty-five patients who received mpMRI prior to radical prostatectomy were included in the study. The imaging protocol included T2, DWI, and DCE. Delineation of tumor regions was performed based on pathological information. First and second order statistics were derived from each structure and for all image modalities. The resulting data were processed with multivariate analysis, using PCA (principal component analysis) and OPLS-DA (orthogonal partial least squares discriminant analysis) for separation of malignant and healthy tissue. PCA showed a clear difference between tumor and healthy regions in the peripheral zone for all investigated images. The predictive ability of the OPLS-DA models increased for all image modalities when first and second order statistics were combined. The predictive value reached a plateau after adding ADC and T2, and did not increase further with the addition of other image information. The present study indicates a distinct difference in the signatures between malign and benign prostate tissue. This is an absolute prerequisite for automatic tumor segmentation, but only the first step in that direction. For the specific identified signature, DCE did not add complementary information to T2 and ADC maps.
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| 2. |
- Dalaryd, Mårten, et al.
(author)
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A Monte Carlo study of a flattening filter-free linear accelerator verified with measurements.
- 2010
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In: Physics in Medicine and Biology. - : IOP Publishing. - 1361-6560 .- 0031-9155. ; 55:23, s. 7333-7344
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Journal article (peer-reviewed)abstract
- A Monte Carlo model of an Elekta Precise linear accelerator has been built and verified by measured data for a 6 and 10 MV photon beam running with and without a flattening filter in the beam line. In this study the flattening filter was replaced with a 6 mm thick copper plate, provided by the linac vendor, in order to stabilize the beam. Several studies have shown that removal of the filter improves some properties of the photon beam, which could be beneficial for radiotherapy treatments. The investigated characteristics of this new beam included output, spectra, mean energy, half value layer and the origin of scattered photons. The results showed an increased dose output per initial electron at the central axis of 1.76 and 2.66 for the 6 and 10 MV beams, respectively. The number of scattered photons from the accelerator head was reduced by (31.7 ± 0.03)% (1 SD) for the 6 MV beam and (47.6 ± 0.02)% for the 10 MV beam. The photon energy spectrum of the unflattened beam was softer compared to a conventional beam and did not vary significantly with the off-axis distance, even for the largest field size (0-20 cm off-axis).
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| 3. |
- Engström, Per, et al.
(author)
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Patient specific QA procedures : In vivo dosimetry
- 2008. - 1
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In: European Guidelines for Quality Assurance in Radiotherapy. : Guidelines for the verification of IMRT. ESTRO Booklet No. 9 - Guidelines for the verification of IMRT. ESTRO Booklet No. 9. ; , s. 84-86
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Book chapter (other academic/artistic)abstract
- In vivo dosimetry of IMRT from Copenhagen University Hospital
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| 4. |
- Engström, Per, et al.
(author)
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Patient specific QA procedures : Pre-treatment verification
- 2008. - 1
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In: European Guidelines for Quality Assurance in Radiotherapy : Guidelines for the verification of IMRT. ESTRO Booklet No. 9 - Guidelines for the verification of IMRT. ESTRO Booklet No. 9. ; , s. 81-81
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Book chapter (other academic/artistic)abstract
- Patient specific QA procedures for IMRT from Lund University Hospital
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| 5. |
- Fetty, Lukas, et al.
(author)
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Investigating conditional GAN performance with different generator architectures, an ensemble model, and different MR scanners for MR-sCT conversion
- 2020
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In: Physics in Medicine and Biology. - : Institute of Physics Publishing (IOPP). - 0031-9155 .- 1361-6560. ; 65:10
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Journal article (peer-reviewed)abstract
- Recent developments in magnetic resonance (MR) to synthetic computed tomography (sCT) conversion have shown that treatment planning is possible without an initial planning CT. Promising conversion results have been demonstrated recently using conditional generative adversarial networks (cGANs). However, the performance is generally only tested on images from one MR scanner, which neglects the potential of neural networks to find general high-level abstract features. In this study, we explored the generalizability of the generator models, trained on a single field strength scanner, to data acquired with higher field strengths. T2-weighted 0.35T MRIs and CTs from 51 patients treated for prostate (40) and cervical cancer (11) were included. 25 of them were used to train four different generators (SE-ResNet, DenseNet, U-Net, and Embedded Net). Further, an ensemble model was created from the four network outputs. The models were validated on 16 patients from a 0.35T MR scanner. Further, the trained models were tested on the Gold Atlas dataset, containing T2-weighted MR scans of different field strengths; 1.5T(7) and 3T(12), and 10 patients from the 0.35T scanner. The sCTs were dosimetrically compared using clinical VMAT plans for all test patients. For the same scanner (0.35T), the results from the different models were comparable on the test set, with only minor differences in the mean absolute error (MAE) (35-51HU body). Similar results were obtained for conversions of 3T GE Signa and the 3T GE Discovery images (40-62HU MAE) for three of the models. However, larger differences were observed for the 1.5T images (48-65HU MAE). The overall best model was found to be the ensemble model. All dose differences were below 1%. This study shows that it is possible to generalize models trained on images of one scanner to other scanners and different field strengths. The best metric results were achieved by the combination of all networks.
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| 6. |
- Fetty, Lukas, et al.
(author)
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Latent space manipulation for high-resolution medical image synthesis via the StyleGAN
- 2020
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In: Zeitschrift für Medizinische Physik. - : Elsevier. - 0939-3889 .- 1876-4436. ; 30:4, s. 305-314
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Journal article (peer-reviewed)abstract
- Introduction: This paper explores the potential of the StyleGAN model as an high-resolution image generator for synthetic medical images. The possibility to generate sample patient images of different modalities can be helpful for training deep learning algorithms as e.g. a data augmentation technique.Methods: The StyleGAN model was trained on Computed Tomography (CT) and T2- weighted Magnetic Resonance (MR) images from 100 patients with pelvic malignancies. The resulting model was investigated with regards to three features: Image Modality, Sex, and Longitudinal Slice Position. Further, the style transfer feature of the StyleGAN was used to move images between the modalities. The root-mean-squard error (RMSE) and the Mean Absolute Error (MAE) were used to quantify errors for MR and CT, respectively.Results: We demonstrate how these features can be transformed by manipulating the latent style vectors, and attempt to quantify how the errors change as we move through the latent style space. The best results were achieved by using the style transfer feature of the StyleGAN (58.7 HU MAE for MR to CT and 0.339 RMSE for CT to MR). Slices below and above an initial central slice can be predicted with an error below 75 HU MAE and 0.3 RMSE within 4 cm for CT and MR, respectively.Discussion: The StyleGAN is a promising model to use for generating synthetic medical images for MR and CT modalities as well as for 3D volumes.
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| 7. |
- Garpebring, Anders, et al.
(author)
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Density Estimation of Grey-Level Co-Occurrence Matrices for Image Texture Analysis
- 2018
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In: Physics in Medicine and Biology. - : Institute of Physics and Engineering in Medicine. - 0031-9155 .- 1361-6560. ; 63:19, s. 9-15
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Journal article (peer-reviewed)abstract
- The Haralick texture features are common in the image analysis literature, partly because of their simplicity and because their values can be interpreted. It was recently observed that the Haralick texture features are very sensitive to the size of the GLCM that was used to compute them, which led to a new formulation that is invariant to the GLCM size. However, these new features still depend on the sample size used to compute the GLCM, i.e. the size of the input image region-of-interest (ROI).The purpose of this work was to investigate the performance of density estimation methods for approximating the GLCM and subsequently the corresponding invariant features.Three density estimation methods were evaluated, namely a piece-wise constant distribution, the Parzen-windows method, and the Gaussian mixture model. The methods were evaluated on 29 different image textures and 20 invariant Haralick texture features as well as a wide range of different ROI sizes.The results indicate that there are two types of features: those that have a clear minimum error for a particular GLCM size for each ROI size, and those whose error decreases monotonically with increased GLCM size. For the first type of features, the Gaussian mixture model gave the smallest errors, and in particular for small ROI sizes (less than about 20×20).In conclusion, the Gaussian mixture model is the preferred method for the first type of features (in particular for small ROIs). For the second type of features, simply using a large GLCM size is preferred.
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| 8. |
- Georg, Dietmar, et al.
(author)
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A practical method to calculate head scatter factors in wedged rectangular and irregular MLC shaped beams for external and internal wedges
- 2004
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In: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 49:20, s. 4689-4700
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Journal article (peer-reviewed)abstract
- Factor based methods for absorbed dose or monitor unit calculations are often based on separate data sets for open and wedged beams. The determination of basic beam parameters can be rather time consuming, unless equivalent square methods are applied. When considering irregular wedged beams shaped with a multileaf collimator, parametrization methods for dosimetric quantities, e.g. output ratios or wedge factors as a function of field size and shape, become even more important. A practical method is presented to derive wedged output ratios in air (S-c,S-w) for any rectangular field and for any irregular MLC shaped beam. This method was based on open field output ratios in air (Sc) for a field with the same collimator setting, and a relation f(w) between S-c,S-w and S-c. The relation f(w) can be determined from measured output ratios in air for a few open and wedged fields including the maximum wedged field size. The function fw and its parametrization were dependent on wedge angle and treatment head design, i.e. they were different for internal and external wedges. The proposed method was tested for rectangular wedged fields on three accelerators with internal wedges (GE, Elekta, BBC) and two accelerators with external wedges (Varian). For symmetric regular beams the average deviation between calculated and measured S-c,S-w/S-c ratios was 0.3% for external wedges and about 0.6% for internal wedges. Maximum deviations of 1.8% were obtained for elongated rectangular fields on the GE and ELEKTA linacs with an internal wedge. The same accuracy was achieved for irregular MLC shaped wedged beams on the accelerators with MLC and internal wedges (GE and Elekta), with an average deviation < 1 % for the fields tested. The proposed method to determine output ratios in air for wedged beams from output ratios of open beams, combined with equivalent square approaches, can be easily integrated in empirical or semi-empirical methods for monitor unit calculations.
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| 9. |
- Georg, Dietmar, et al.
(author)
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Clinical evaluation of monitor unit software and the application of action levels
- 2007
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In: Radiotherapy and Oncology. - : Elsevier BV. - 0167-8140 .- 1879-0887. ; 85:2, s. 306-315
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Journal article (peer-reviewed)abstract
- PURPOSE: The aim of this study was the clinical evaluation of an independent dose and monitor unit verification (MUV) software which is based on sophisticated semi-analytical modelling. The software was developed within the framework of an ESTRO project. Finally, consistent handling of dose calculation deviations applying individual action levels is discussed. MATERIALS AND METHODS: A Matlab-based software ("MUV") was distributed to five well-established treatment centres in Europe (Vienna, Graz, Basel, Copenhagen, and Umeå) and evaluated as a quality assurance (QA) tool in clinical routine. Results were acquired for 226 individual treatment plans including a total of 815 radiation fields. About 150 beam verification measurements were performed for a portion of the individual treatment plans, mainly with time variable fluence patterns. The deviations between dose calculations performed with a treatment planning system (TPS) and the MUV software were scored with respect to treatment area, treatment technique, geometrical depth, radiological depth, etc. RESULTS: In general good agreement was found between calculations performed with the different TPSs and MUV, with a mean deviation per field of 0.2+/-3.5% (1 SD) and mean deviations of 0.2+/-2.2% for composite treatment plans. For pelvic treatments less than 10% of all fields showed deviations larger than 3%. In general, when using the radiological depth for verification calculations the results and the spread in the results improved significantly, especially for head-and-neck and for thorax treatments. For IMRT head-and-neck beams, mean deviations between MUV and the local TPS were -1.0+/-7.3% for dynamic, and -1.3+/-3.2% for step-and-shoot IMRT delivery. For dynamic IMRT beams in the pelvis good agreement was obtained between MUV and the local TPS (mean: -1.6+/-1.5%). Treatment site and treatment technique dependent action levels between +/-3% and +/-5% seem to be clinically realistic if a radiological depth correction is performed, even for dynamic wedges and IMRT. CONCLUSION: The software MUV is well suited for patient specific treatment plan QA applications and can handle all currently available treatment techniques that can be applied with standard linear accelerators. The highly sophisticated dose calculation model implemented in MUV allows investigation of systematic TPS deviations by performing calculations in homogeneous conditions
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| 10. |
- Georg, Dietmar, et al.
(author)
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Current status and future perspective of flattening filter free photon beams
- 2011
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In: Medical Physics. - : Wiley. - 0094-2405. ; 38:3, s. 1280-1293
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Journal article (peer-reviewed)abstract
- Purpose: Flattening filters (FFs) have been considered as an integral part of the treatment head of a medical accelerator for more than 50 years. The reasons for the longstanding use are, however, historical ones. Advanced treatment techniques, such as stereotactic radiotherapy or intensity modulated radiotherapy have stimulated the interest in operating linear accelerators in a flattening filter free (FFF) mode. The current manuscript reviews treatment head physics of FFF beams, describes their characteristics and the resulting potential advantages in their medical use, and closes with an outlook. Methods: A number of dosimetric benefits have been determined for FFF beams, which range from increased dose rate and dose per pulse to favorable output ratio in-air variation with field size, reduced energy variation across the beam, and reduced leakage and out-of-field dose, respectively. Finally, the softer photon spectrum of unflattened beams has implications on imaging strategies and radiation protection. Results: The dosimetric characteristics of FFF beams have an effect on treatment delivery, patient comfort, dose calculation accuracy, beam matching, absorbed dose determination, treatment planning, machine specific quality assurance, imaging, and radiation protection. When considering conventional C-arm linacs in a FFF mode, more studies are needed to specify and quantify the clinical advantages, especially with respect to treatment plan quality and quality assurance. Conclusions: New treatment units are already on the market that operate without a FF or can be operated in a dedicated clinical FFF mode. Due to the convincing arguments of removing the FF, it is expected that more vendors will offer dedicated treatment units for advanced photon beam therapy in the near future. Several aspects related to standardization, dosimetry, treatment planning, and optimization need to be addressed in more detail in order to facilitate the clinical implementation of unflattened beams. (C) 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3554643]
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