The heart is important in radiotherapy either as target or organ at risk. Quantitative T1 and T2 cardiac magnetic resonance imaging (qMRI) may aid in target definition for cardiac radioablation, and ...imaging biomarker for cardiotoxicity assessment. Hybrid MR-linac devices could facilitate daily cardiac qMRI of the heart in radiotherapy. The aim of this work was therefore to enable cardiac-synchronized T1 and T2 mapping on a 1.5 T MR-linac and test the reproducibility of these sequences on phantoms and in vivo between the MR-linac and a diagnostic 1.5 T MRI scanner.
Cardiac-synchronized MRI was performed on the MR-linac using a wireless peripheral pulse-oximeter unit. Diagnostically used T1 and T2 mapping sequences were acquired twice on the MR-linac and on a 1.5 T MR-simulator for a gel phantom and 5 healthy volunteers in breath-hold. Phantom T1 and T2 values were compared to gold-standard measurements and percentage errors (PE) were computed, where negative/positive PE indicate underestimations/overestimations. Manually selected regions-of-interest were used for in vivo intra/inter scanner evaluation.
Cardiac-synchronized T1 and T2 qMRI was enabled after successful hardware installation on the MR-linac. From the phantom experiments, the measured T1/T2 relaxation times had a maximum percentage error (PE) of −4.4%/−8.8% on the MR-simulator and a maximum PE of −3.2%/+8.6% on the MR-linac. Mean T1/T2 of the myocardium were 1012±34/51±2 ms on the MR-simulator and 1034±42/51±1 ms on the MR-linac.
Accurate cardiac-synchronized T1 and T2 mapping is feasible on a 1.5 T MR-linac and might enable novel plan adaptation workflows and cardiotoxicity assessments.
Diffusion weighted (DW) MRI may facilitate target volume delineation for head-and-neck (HN) radiation treatment planning. In this study we assessed the use of a dedicated, geometrically accurate, ...DW-MRI sequence for target volume delineation. The delineations were compared with semi-automatic segmentations on 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) images and evaluated for interobserver variation.
Fifteen HN cancer patients underwent both DW-MRI and FDG-PET for RT treatment planning. Target delineation on DW-MRI was performed by three observers, while for PET a semi-automatic segmentation was performed using a Gaussian mixture model. For interobserver variation and intermodality variation, volumes, overlap metrics and Hausdorff distances were calculated from the delineations.
The median volumes delineated by the three observers on DW-MRI were 10.8, 10.5 and 9.0 cm3 respectively, and was larger than the median PET volume (8.0 cm3). The median conformity index of DW-MRI for interobserver variation was 0.73 (range 0.38–0.80). Compared to PET, the delineations on DW-MRI by the three observers showed a median dice similarity coefficient of 0.71, 0.69 and 0.72 respectively. The mean Hausdorff distance was small with median (range) distances between PET and DW-MRI of 2.3 (1.5–6.8), 2.5 (1.6–6.9) and 2.0 (1.35–7.6) mm respectively. Over all patients, the median 95th percentile distances were 6.0 (3.0–13.4), 6.6 (4.0–24.0) and 5.3 (3.4–26.0) mm.
Using a dedicated DW-MRI sequence, target volumes could be defined with good interobserver agreement and a good overlap with PET. Target volume delineation using DW-MRI is promising in head-and-neck radiotherapy, combined with other modalities, it can lead to more precise target volume delineation.
PurposeThe diffusion‐weighted SPLICE (split acquisition of fast spin‐echo signals) sequence employs split‐echo rapid acquisition with relaxation enhancement (RARE) readout to provide images almost ...free of geometric distortions. However, due to the varying T2$$ {}_2 $$‐weighting during k‐space traversal, SPLICE suffers from blurring. This work extends a method for controlling the spatial point spread function (PSF) while optimizing the signal‐to‐noise ratio (SNR) achieved by adjusting the flip angles in the refocusing pulse train of SPLICE.MethodsAn algorithm based on extended phase graph (EPG) simulations optimizes the flip angles by maximizing SNR for a flexibly chosen predefined target PSF that describes the desired k‐space density weighting and spatial resolution. An optimized flip angle scheme and a corresponding post‐processing correction filter which together achieve the target PSF was tested by healthy subject brain imaging using a clinical 1.5 T scanner.ResultsBrain images showed a clear and consistent improvement over those obtained with a standard constant flip angle scheme. SNR was increased and apparent diffusion coefficient estimates were more accurate. For a modified Hann k‐space weighting example, considerable benefits resulted from acquisition weighting by flip angle control.ConclusionThe presented flexible method for optimizing SPLICE flip angle schemes offers improved MR image quality of geometrically accurate diffusion‐weighted images that makes the sequence a strong candidate for radiotherapy planning or stereotactic surgery.
Purpose
The diffusion‐weighted SPLICE (split acquisition of fast spin‐echo signals) sequence employs split‐echo rapid acquisition with relaxation enhancement (RARE) readout to provide images almost ...free of geometric distortions. However, due to the varying T2$$ {}_2 $$‐weighting during k‐space traversal, SPLICE suffers from blurring. This work extends a method for controlling the spatial point spread function (PSF) while optimizing the signal‐to‐noise ratio (SNR) achieved by adjusting the flip angles in the refocusing pulse train of SPLICE.
Methods
An algorithm based on extended phase graph (EPG) simulations optimizes the flip angles by maximizing SNR for a flexibly chosen predefined target PSF that describes the desired k‐space density weighting and spatial resolution. An optimized flip angle scheme and a corresponding post‐processing correction filter which together achieve the target PSF was tested by healthy subject brain imaging using a clinical 1.5 T scanner.
Results
Brain images showed a clear and consistent improvement over those obtained with a standard constant flip angle scheme. SNR was increased and apparent diffusion coefficient estimates were more accurate. For a modified Hann k‐space weighting example, considerable benefits resulted from acquisition weighting by flip angle control.
Conclusion
The presented flexible method for optimizing SPLICE flip angle schemes offers improved MR image quality of geometrically accurate diffusion‐weighted images that makes the sequence a strong candidate for radiotherapy planning or stereotactic surgery.
In a typical MR session, several contrasts are acquired. Due to the sequential nature of the data acquisition process, the patient may experience some discomfort at some point during the session, and ...start moving. Hence, it is quite common to have MR sessions where some contrasts are well-resolved, while other contrasts exhibit motion artifacts. Instead of repeating the scans that are corrupted by motion, we introduce a reference-guided retrospective motion correction scheme that takes advantage of the motion-free scans, based on a generalized rigid registration routine.
We focus on various existing clinical 3D brain protocols at 1.5 Tesla MRI based on Cartesian sampling. Controlled experiments with three healthy volunteers and three levels of motion are performed.
Radiological inspection confirms that the proposed method consistently ameliorates the corrupted scans. Furthermore, for the set of specific motion tests performed in this study, the quality indexes based on PSNR and SSIM shows only a modest decrease in correction quality as a function of motion complexity.
While the results on controlled experiments are positive, future applications to patient data will ultimately clarify whether the proposed correction scheme satisfies the radiological requirements.
To be able to examine dynamic and detailed brain functions, the spatial and temporal resolution of 7 T MRI needs to improve. In this study, it was investigated whether submillimeter multishot 3D EPI ...fMRI scans, acquired with high‐density receive arrays, can benefit from a 2D CAIPIRINHA sampling pattern, in terms of noise amplification (g‐factor), temporal SNR and fMRI sensitivity. High‐density receive arrays were combined with a shot‐selective 2D CAIPIRINHA implementation for multishot 3D EPI sequences at 7 T. In this implementation, in contrast to conventional inclusion of extra kz gradient blips, specific EPI shots are left out to create a CAIPIRINHA shift and reduction of scan time. First, the implementation of the CAIPIRINHA sequence was evaluated with a standard receive setup by acquiring submillimeter whole brain T2*‐weighted anatomy images. Second, the CAIPIRINHA sequence was combined with high‐density receive arrays to push the temporal resolution of submillimeter 3D EPI fMRI scans of the visual cortex. Results show that the shot‐selective 2D CAIPIRINHA sequence enables a reduction in scan time for 0.5 mm isotropic 3D EPI T2*‐weighted anatomy scans by a factor of 4 compared with earlier reports. The use of the 2D CAIPIRINHA implementation in combination with high‐density receive arrays, enhances the image quality of submillimeter 3D EPI scans of the visual cortex at high acceleration as compared to conventional SENSE. Both the g‐factor and temporal SNR improved, resulting in a method that is more sensitive to the fMRI signal. Using this method, it is possible to acquire submillimeter single volume 3D EPI scans of the visual cortex in a subsecond timeframe. Overall, high‐density receive arrays in combination with shot‐selective 2D CAIPIRINHA for 3D EPI scans prove to be valuable for reducing the scan time of submillimeter MRI acquisitions.
This study investigated whether submillimeter 3D EPI fMRI scans acquired with high‐density receive arrays can still benefit from a CAIPI sampling pattern, in terms of scan time per volume and (temporal) SNR. A shot‐selective 2D CAIPIRINHA acceleration scheme for multishot 3D EPI scans was implemented and evaluated with submillimeter 3D T2*‐weighted anatomical scans. When combining this implementation with high‐density receive coil arrays at 7 T, it enabled substantial reduction in scan time for submillimeter fMRI scans of the visual cortex.
Abstract Background and purpose The goal of this study was to improve target definition by deriving modality-specific margins for clinical target volumes (CTV) for laryngeal and hypopharyngeal cancer ...on CT, MRI and 18-FDG-PET. Material and methods Twenty-five patients with T3/T4 laryngeal/hypopharyngeal cancer underwent CT, MRI and 18-FDG-PET scans before laryngectomy. HE-sections were obtained from the surgical specimen and tumor was delineated (tumorHE ). The GTVs on CT and MRI were delineated in consensus. PET-based GTVs were automatically segmented. The three-dimensionally reconstructed specimen was registered to the various images. Modality-specific CTV margins were derived and added to the GTVs to achieve adequate tumor coverage. The resulting CTVs were compared with each other, to tumorHE , and to CTVCT10 constructed on CT with the clinical margin of 10 mm. Results CTV margins of 4.3 mm (CT), 6.1 mm (MRI) and 5.2 mm (PET) were needed to achieve adequate tumor coverage. The median volumes of the resulting modality-specific CTVs were 44 ml (CT), 48 ml (MRI) and 39 ml (PET), while the CTV10mm was 80 ml. Conclusion For laryngohypopharyngeal tumors, 45–52% target volume reduction compared with CTV10mm is achievable when modality-specific CTV margins are used. PET-based CTVs were significantly smaller compared to CT- and MRI-based CTVs.
•GTV delineations based on anatomical MRI overestimated the tumor volume by 95%•Geometrically accurate DW-MRI increases tumor delineation accuracy.•DW-MRI reduces the CTV volumes, while maintaining ...high tumor coverage.
This study aims to determine the added value of a geometrically accurate diffusion-weighted (DW-) MRI sequence on the accuracy of gross tumor volume (GTV) delineations, using pathological tumor delineations as a ground truth.
Sixteen patients with laryngeal or hypopharyngeal carcinoma were included. After total laryngectomy, the specimen was cut into slices. Photographs of these slices were stacked to create a 3D digital specimen reconstruction, which was registered to the in vivo imaging. The pathological tumor (tumorHE) was delineated on the specimen reconstruction.
Six observers delineated all tumors twice: once with only anatomical MR imaging, and once (a few weeks later) when DW sequences were also provided. The majority voting delineation of session one (GTVMRI) and session two (GTVDW-MRI), as well as the clinical target volumes (CTVs), were compared to the tumorHE.
The mean tumorHE volume was 11.1 cm3, compared to a mean GTVMRI volume of 18.5 cm3 and a mean GTVDW-MRI volume of 15.7 cm3. The median sensitivity (tumor coverage) was comparable between sessions: 0.93 (range: 0.61–0.99) for the GTVMRI and 0.91 (range: 0.53–1.00) for the GTVDW-MRI.
The CTV volume also decreased when DWI was available, with a mean CTVMR of 47.1 cm3 and a mean CTVDW-MRI of 41.4 cm3. Complete tumor coverage was achieved in 15 and 14 tumors, respectively.
GTV delineations based on anatomical MR imaging tend to overestimate the tumor volume. The availability of the geometrically accurate DW sequence reduces the GTV overestimation and thereby CTV volumes, while maintaining acceptable tumor coverage.
Purpose
Diffusion‐weighted (DW) MRI, showing high contrast between tumor and background tissue, is a promising technique in radiotherapy for tumor delineation. However, its use for head‐and‐neck ...patients is hampered by poor geometric accuracy in conventional echo planar imaging (EPI) DW‐MRI. An alternative turbo spin echo sequence, DW‐SPLICE, is implemented and demonstrated in patients.
Methods
The DW‐SPLICE sequence was implemented on a 3.0 T system and evaluated in 10 patients. The patients were scanned in treatment position, using a customized head support and immobilization mask. Image distortions were quantified at the gross tumor volume (GTV) using field map analysis. The apparent diffusion coefficient (ADC) was evaluated using an ice water phantom.
Results
The DW images acquired by DW‐SPLICE showed no image distortions. Field map analysis at the gross tumor volumes resulted in a median distortion of 0.2 mm for DW‐SPLICE, whereas for the conventional method this was 7.2 mm. ADC values, measured using an ice water phantom were in accordance with literature values.
Conclusions
The implementation of DW‐SPLICE allows for diffusion‐weighted imaging of patients in treatment position with excellent geometrical accuracy. The images can be used to facilitate target volume delineation in RT treatment planning.
Abstract Background While surgery remains the cornerstone of rectal cancer treatment, organ-preservation is upcoming. Therefore, neo-adjuvant treatment should be optimized. By escalating doses, ...response can be increased. To limit toxicity of boost, accurate gross tumor volume (GTV) definition is required. MRI, especially undeformed fast spin echo diffusion-weighted MRI (DWI), looks promising for delineation. However, inconsistencies between observers should be quantified before clinical implementation. We aim to find which MRI sequence (T2w, DWI or combination) is optimal and clinically useful for GTV definition by evaluating inter-observer agreement. Methods Locally advanced rectal cancer patients (tumors <10 cm from anal verge) were scanned on 3T MRI transverse T2w and DWI ( b = 800 s/mm2 ). Three independent observers delineated T2w, DWI and combination (Combi) after training-set. Volumes, conformity index (CI), and maximum Hausdorff distance (HD) were calculated between any observer-pair per patient per modality. Results Twenty-four consecutive patients were included. One patient had cT2 (4.2%), 19 cT3 (79.1%) and 4 cT4 (16.7%), with 2 clinical node negative (8.3%), 4 cN1 (16.7%), and 18 cN2 (75.0%) on MRI. From 24 patients, 70 sequences were available (24x T2, 23x DWI, and 23x Combi). Between observers, no significant volume differences were observed per modality. T2 showed significantly largest volumes compared to DWI (mean difference 19.85 ml, SD 17.42, p < 0.0001) and Combi (mean difference 7.16 ml, SD 11.58, p < 0.0001). Mean CI was 0.70, 0.71 and 0.69 for T2, DWI and Combi respectively ( p > 0.61). Average HD was largest on T2 (18.60 mm, max 31.40 mm, min 9.20 mm). Discussion Delineation on DWI resulted in delineation of the smallest volumes with similar consistency and mean distances, but with slightly lower Hausdorff distances compared to T2 and Combi. However, with lack of a gold standard it remains difficult to establish if delineations also represent true tumor. Study strengths were DWI adaptation to exclude geometrical distortions and training-set. DWI shows great potential for delineation purposes as long as sufficient experience exists and geometrical distortions are eliminated.
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