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Dosimetric Impact of MRI Distortions: A Study on Head and Neck Cancers
Umeå universitet, Medicinska fakulteten, Institutionen för strålningsvetenskaper, Radiofysik.ORCID-id: 0000-0003-3217-3208
Umeå universitet, Medicinska fakulteten, Institutionen för strålningsvetenskaper, Radiofysik.ORCID-id: 0000-0001-7539-2262
Umeå universitet, Medicinska fakulteten, Institutionen för strålningsvetenskaper, Radiofysik.
Umeå universitet, Medicinska fakulteten, Institutionen för strålningsvetenskaper, Onkologi.
Visa övriga samt affilieringar
2019 (Engelska)Ingår i: International Journal of Radiation Oncology, Biology, Physics, ISSN 0360-3016, E-ISSN 1879-355X, Vol. 103, nr 4, s. 994-1003Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Purpose: To evaluate the effect of magnetic resonance (MR) imaging (MRI) geometric distortions on head and neck radiation therapy treatment planning (RTP) for an MRI-only RTP. We also assessed the potential benefits of patient-specific shimming to reduce the magnitude of MR distortions for a 3-T scanner.

Methods and Materials: Using an in-house Matlab algorithm, shimming within entire imaging volumes and user-defined regions of interest were simulated. We deformed 21 patient computed tomography (CT) images with MR distortion fields (gradient nonlinearity and patient-induced susceptibility effects) to create distorted CT (dCT) images using bandwidths of 122 and 488 Hz/mm at 3 T. Field parameters from volumetric modulated arc therapy plans initially optimized on dCT data sets were transferred to CT data to compute a new plan. Both plans were compared to determine the impact of distortions on dose distributions.

Results: Shimming across entire patient volumes decreased the percentage of voxels with distortions of more than 2 mm from 15.4% to 2.0%. Using the user-defined region of interest (ROI) shimming strategy, (here the Planning target volume (PTV) was the chosen ROI volume) led to increased geometric for volumes outside the PTV, as such voxels within the spinal cord with geometric shifts above 2 mm increased from 11.5% to 32.3%. The worst phantom-measured residual system distortions after 3-dimensional gradient nonlinearity correction within a radial distance of 200 mm from the isocenter was 2.17 mm. For all patients, voxels with distortion shifts of more than 2 mm resulting from patient-induced susceptibility effects were 15.4% and 0.0% using bandwidths of 122 Hz/mm and 488 Hz/mm at 3 T. Dose differences between dCT and CT treatment plans in D-50 at the planning target volume were 0.4% +/- 0.6% and 0.3% +/- 0.5% at 122 and 488 Hz/mm, respectively.

Conclusions: The overall effect of MRI geometric distortions on data used for RTP was minimal. Shimming over entire imaging volumes decreased distortions, but user-defined subvolume shimming introduced significant errors in nearby organs and should probably be avoided.

Ort, förlag, år, upplaga, sidor
Elsevier, 2019. Vol. 103, nr 4, s. 994-1003
Nationell ämneskategori
Radiologi och bildbehandling
Identifikatorer
URN: urn:nbn:se:umu:diva-157192DOI: 10.1016/j.ijrobp.2018.11.037ISI: 000459153600031PubMedID: 30496879Scopus ID: 2-s2.0-85061601182OAI: oai:DiVA.org:umu-157192DiVA, id: diva2:1304925
Tillgänglig från: 2019-04-15 Skapad: 2019-04-15 Senast uppdaterad: 2023-10-13Bibliografiskt granskad
Ingår i avhandling
1. Quality assurance for magnetic resonance imaging (MRI) in radiotherapy
Öppna denna publikation i ny flik eller fönster >>Quality assurance for magnetic resonance imaging (MRI) in radiotherapy
2019 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
Alternativ titel[sv]
Kvalitetssäkring av magnetisk resonanstomografi (MRI/MRT) inom radioterapi
Abstract [en]

The use of Magnetic Resonance Imaging (MRI) in the radiotherapy (RT) treatment planning workflow is increasing. MRI offers superior soft-tissue contrast compared to Computed Tomography (CT) and therefore improves the accuracy in target volume definitions. There are, however concerns with inherent geometric distortions from system- (gradient nonlinearities and main magnetic field inhomogeneities) and patient-related sources (magnetic susceptibility effect and chemical shift). The lack of clearly defined quality assurance (QA) procedures has also raised questions on the ability of current QA protocols to detect common image quality degradations under radiotherapy settings. To fully implement and take advantage of the benefits of MRI in radiotherapy, these concerns need to be addressed.

In Papers I and II, the dosimetric impact of MR distortions was investigated. Patient CTs (CT) were deformed with MR distortion vector fields (from the residual system distortions after correcting for gradient nonlinearities and patient-induced susceptibility distortions) to create distorted CT (dCT) images. Field parameters from volumetric modulated arc therapy (VMAT) treatment plans initially optimized on dCT data sets were transferred to CT data to compute new treatment plans. Data from 19 prostate and 21 head and neck patients were used for the treatment planning. The dCT and CT treatment plans were compared to determine the impact of distortions on dose distributions. No clinically relevant dose differences between distorted CT and original CT treatment plans were found. Mean dose differences were < 1.0% and < 0.5% at the planning target volume (PTV) for the head and neck, and prostate treatment plans, respectively. 

Strategies to reduce geometric distortions were also evaluated in Papers I and II. Using the vendor-supplied gradient non-linearity correction algorithm reduced overall distortions to less than half of the original value. A high acquisition bandwidth of 488 Hz/pixel (Paper I) and 488 Hz/mm (Paper II) kept the mean geometric distortions at the delineated structures below 1 mm. Furthermore, a patient-specific active shimming method implemented in Paper II significantly reduced the number of voxels with distortion shifts > 2 mm from 15.4% to 2.0%.

B0 maps from patient-induced magnetic field inhomogeneities obtained through direct measurements and by simulations that used MR-generated synthetic CT (sCT) data were compared in Paper III. The validation showed excellent agreement between the simulated and measured B0 maps.

In Paper IV, the ability of current QA methods to detect common MR image quality degradations under radiotherapy settings were investigated. By evaluating key image quality parameters, the QA protocols were found to be sensitive to some of the introduced degradations. However, image quality issues such as those caused by RF coil failures could not be adequately detected.

In conclusion, this work has shown the feasibility of using MRI data for radiotherapy treatment planning as distortions resulted in a dose difference of less than 1% between distorted and undistorted images. The simulation software can be used to produce accurate B0 maps, which could then be used as the basis for the effective correction of patient-induced field inhomogeneity distortions and for the QA verification of sCT data. Furthermore, the analysis of the strengths and weaknesses in current QA tools for MRI in RT contribute to finding better methods to efficiently identify image quality errors.

Ort, förlag, år, upplaga, sidor
Umeå: Medical Faculty, Umeå University, 2019. s. 78
Serie
Umeå University medical dissertations, ISSN 0346-6612 ; 2057
Nyckelord
magnetic resonance imaging, MRI, radiotherapy, RT, geometric distortions, magnetic susceptibility, B0 maps, quality assurance
Nationell ämneskategori
Radiologi och bildbehandling
Forskningsämne
radiofysik
Identifikatorer
urn:nbn:se:umu:diva-164771 (URN)978-91-7855-130-9 (ISBN)
Disputation
2019-11-29, Hörsal 933, byggnad 3, Norrlands universitetssjukhus, Umeå, 09:00 (Engelska)
Opponent
Handledare
Forskningsfinansiär
Cancerforskningsfonden i Norrland, AMP 18-905
Anmärkning

PhD Study of Author partially funded by The Schlumberger Faculty for the Future Foundation (FFTF)

Tillgänglig från: 2019-11-08 Skapad: 2019-10-31 Senast uppdaterad: 2023-09-14Bibliografiskt granskad
2. Geometric distortions in MRI based radiotherapy and PET/MRI
Öppna denna publikation i ny flik eller fönster >>Geometric distortions in MRI based radiotherapy and PET/MRI
2022 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
Alternativ titel[sv]
Geometriska distortioner i MR baserad strålterapi och PET/MR
Abstract [en]

Magnetic resonance imaging (MRI) offers high soft-tissue contrast compared to computed tomography (CT). This contrast is helpful in many cases, not least for delineating tumours for radiotherapy treatment, and has led to increasing use in radiotherapy treatment planning (RTP).

When RTP is based on CT images, the treatment planning system can get the approximate electron density of the tissues from the electron density equivalent information that constitutes the CT images. This information is needed to calculate the dose to the patient from radiotherapy. Therefore, for an MR-only workflow, the MRI image must be transformed into information that can yield electron density information. The predominant way is to convert the MRI images into CT-like images, also known as substitute CT (sCT).

Positron emission tomography (PET) imaging can benefit from being combined with anatomical imaging, and the PET/CT hybrid machine is well established. The soft tissue contrast properties of the MRI images are also valuable for a PET/MRI hybrid system. However, it also adds the option for simultaneous acquisition of PET and anatomical (MRI) images which is not feasible with CT images for a PET/CT system. However, the PET/MRI combination is more technically challenging. While most of the concerns have been solved or mitigated, and PET/MRI systems have been commercially available for some years, there are still outstanding issues. The attenuation maps used in the reconstruction of PET acquisitions are one of the concerns that have yet to be solved entirely. These attenuation maps in the PET/MRI systems are approximations where, e.g., bone is not fully accounted for in all parts of the body.

The MRI images suffer from geometric distortions dependent on the MRI scanner as well as the imaged patient itself. These distortions can affect the sCT conversion for RTP and the attenuation maps for PET reconstruction.

This thesis aimed to investigate the size of the geometric MRI distortions for different settings on the MRI scanner and their effect on the resulting RTP and reconstructed PET images. Such information can aid in optimising the MRI imaging for different purposes. It can also give some information needed to determine tests to run in a quality assurance (QA) regime.

In Paper I, we studied the machine-dependent MRI gradient-field nonlinearity distortions and their effect on PET reconstruction. We simulated different levels of incomplete corrections for gradient-field nonlinearities in CT images from PET/CT acquisitions. The resulting distorted images were then used for rerunning the reconstruction of PET data, and the effect on reconstructed standardised uptake value (SUV) was studied. We found that residual gradient-field nonlinearity dependent geometrical distortions of ±2.3 mm at 15 cm radius from the scanner isocenter lead to SUV quantification errors below 5%. This is also below the test-retest variability caused by instrumentation and intra-patient factors for PET/CT systems.

In Paper II, we developed a method for simulating the patient-induced susceptibility effect based on CT images. The method consisted of converting the CT images to magnetic susceptibility maps. These magnetic susceptibility maps were then used in the simulation by calculating the local shifts in the main magnetic field (B0). From these local shifts in B0, a displacement map was calculated, and this was, in turn, applied to the original CT images. The simulation was validated through comparisons between the simulation and analytical results for both a homogeneous sphere and a homogeneous cylinder. This method was tested on a set of eight prostate cancer patients. We found that setting the frequency encoding bandwidth to a minimum of twice the water-fat shift would keep the maximum distortion from the patient-induced susceptibility effect below 1 pixel. However, the required frequency encoding bandwidth was shown to be dependent on the imaged area, and lower bandwidth could, e.g., be used for the pelvic area.

The simulation method from Paper II was then used in Paper III, where we investigated the dosimetric impact of residual MRI system distortions, patient-induced susceptibility effects and patient-specific shimming. The latter was simulated using an in-house Matlab algorithm. The residual system distortions were determined using phantom measurements. These distortions were then combined with a simulated patient-induced susceptibility effect and patient-specific shimming. The combined distortions and patient-specific shimming were then applied to patient CT images. The distorted patient images were then used for RTP, and the resulting treatment plan was transferred to the original patient CT datasets and recalculated. This allowed for isolating the effect of the applied distortions on the dose distribution. We concluded that the dosimetric impact of MRI distortions within the target volume and nearby organs at risk is small for high bandwidth spin echo sequences. We also saw a worsening in field variations for the user-defined region ofinterest shimming.

Paper IV presented a proof of concept for patient-specific QA of sCTs and attenuation maps. In this study, we compared measured B0-maps with ones simulated using Paper II’s method. The simulations were based on sCT images from MRI acquisitions from the same imaging session as the measured B0-maps. This method shows potential for identifying errors or problematic areas of sCTs and attenuation maps. It should also be feasible to make the method fast enough to use while the patient is still in the scanner so that images could be retaken without having to recall thepatient if a problem is detected.

This work has contributed to the knowledge and methods needed for the necessary considerations for optimisation and setting up a QA protocol aimed at PET/MRI and MR-only radiotherapy.

Ort, förlag, år, upplaga, sidor
Umeå: Umeå universitet, 2022. s. 48
Serie
Umeå University medical dissertations, ISSN 0346-6612 ; 2195
Nyckelord
Magnetic resonance imaging, MRI, PET/MR, PET/MRI, medical imaging, radiotherapy, geometrical distortions, MR-only, MRI-only
Nationell ämneskategori
Radiologi och bildbehandling
Forskningsämne
radiofysik
Identifikatorer
urn:nbn:se:umu:diva-199795 (URN)978-91-7855-831-5 (ISBN)978-91-7855-832-2 (ISBN)
Disputation
2022-10-28, Hörsal D, byggnad 1D T9, Norrlands universitetssjukhus, Umeå, 09:00 (Engelska)
Opponent
Handledare
Tillgänglig från: 2022-10-07 Skapad: 2022-09-29 Senast uppdaterad: 2022-09-30Bibliografiskt granskad

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