Evaluating the accuracy of geometrical distortion correction of magnetic resonance images for use in intracranial brain tumor radiotherapy

Ultrasound-guided breast biopsy using an adapted automated cone-based ultrasound scanner: a feasibility study

BACKGROUND

Among available breast biopsy techniques, ultrasound (US)-guided biopsy is preferable because it is relatively inexpensive and provides live imaging feedback. The availability of magnetic resonance imaging (MRI)-3D US image fusion would facilitate US-guided biopsy even for US occult lesions to reduce the need for expensive and time-consuming MRI-guided biopsy. In this paper, we propose a novel Automated Cone-based Breast Ultrasound Scanning and Biopsy System (ACBUS-BS) to scan and biopsy breasts of women in prone position. It is based on a previously developed system, called ACBUS, that facilitates MRI-3D US image fusion imaging of the breast employing a conical container filled with coupling medium.

PURPOSE

The purpose of this study was to introduce the ABCUS-BS system and demonstrate its feasibility for biopsy of US occult lesions.

METHOD

The biopsy procedure with the ACBUS-BS comprises four steps: target localization, positioning, preparation, and biopsy. The biopsy outcome can be impacted by 5 types of errors: due to lesion segmentation, MRI-3D US registration, navigation, lesion tracking during repositioning, and US inaccuracy (due to sound speed difference between the sample and the one used for image reconstruction). For the quantification, we use a soft custom-made polyvinyl alcohol phantom (PVA) containing eight lesions (three US-occult and five US-visible lesions of 10 mm in diameter) and a commercial breast mimicking phantom with a median stiffness of 7.6 and 28 kPa, respectively. Errors of all types were quantified using the custom-made phantom. The error due to lesion tracking was also quantified with the commercial phantom. Finally, the technology was validated by biopsying the custom-made phantom and comparing the size of the biopsied material to the original lesion size. The average size of the 10-mm-sized lesions in the biopsy specimen was 7.00 ± 0.92 mm (6.33 ± 1.16 mm for US occult lesions, and 7.40 ± 0.55 mm for US-visible lesions).

RESULTS

For the PVA phantom, the errors due to registration, navigation, lesion tracking during repositioning, and US inaccuracy were 1.33, 0.30, 2.12, and 0.55 mm. The total error was 4.01 mm. For the commercial phantom, the error due to lesion tracking was estimated at 1.10 mm, and the total error was 4.11 mm. Given these results, the system is expected to successfully biopsy lesions larger than 8.22 mm in diameter. Patient studies will have to be carried out to confirm this in vivo.

CONCLUSION

The ACBUS-BS facilitates US-guided biopsy of lesions detected in pre-MRI and therefore might offer a low-cost alternative to MRI-guided biopsy. We demonstrated the feasibility of the approach by successfully taking biopsies of five US-visible and three US-occult lesions embedded in a soft breast-shaped phantom.

Adoption of Hybrid MRI-Linac Systems for the Treatment of Brain Tumors: A Systematic Review of the Current Literature Regarding Clinical and Technical Features

BACKGROUND

Possible advantages of magnetic resonance (MR)-guided radiation therapy (MRgRT) for the treatment of brain tumors include improved definition of treatment volumes and organs at risk (OARs) that could allow margin reductions, resulting in limited dose to the OARs and/or dose escalation to target volumes. Recently, hybrid systems integrating a linear accelerator and an magnetic resonance imaging (MRI) scan (MRI-linacs, MRL) have been introduced, that could potentially lead to a fully MRI-based treatment workflow.

METHODS

We performed a systematic review of the published literature regarding the adoption of MRL for the treatment of primary or secondary brain tumors (last update November 3, 2022), retrieving a total of 2487 records; after a selection based on title and abstracts, the full text of 74 articles was analyzed, finally resulting in the 52 papers included in this review.

RESULTS AND DISCUSSION

Several solutions have been implemented to achieve a paradigm shift from CT-based radiotherapy to MRgRT, such as the management of geometric integrity and the definition of synthetic CT models that estimate electron density. Multiple sequences have been optimized to acquire images with adequate quality with on-board MR scanner in limited times. Various sophisticated algorithms have been developed to compensate the impact of magnetic field on dose distribution and calculate daily adaptive plans in a few minutes with satisfactory dosimetric parameters for the treatment of primary brain tumors and cerebral metastases. Dosimetric studies and preliminary clinical experiences demonstrated the feasibility of treating brain lesions with MRL.

CONCLUSIONS

The adoption of an MRI-only workflow is feasible and could offer several advantages for the treatment of brain tumors, including superior image quality for lesions and OARs and the possibility to adapt the treatment plan on the basis of daily MRI. The growing body of clinical data will clarify the potential benefit in terms of toxicity and response to treatment.

Evaluation of the ability of the Brainlab Elements Cranial Distortion Correction algorithm to correct clinically relevant MRI distortions for cranial SRT

PURPOSE

Cranial stereotactic radiotherapy (SRT) requires highly accurate lesion delineation. However, MRI can have significant inherent geometric distortions. We investigated how well the Elements Cranial Distortion Correction algorithm of Brainlab (Munich, Germany) corrects the distortions in MR image-sets of a phantom and patients.

METHODS

A non-distorted reference computed tomography image-set of a CIRS Model 603-GS (CIRS, Norfolk, VA, USA) phantom was acquired. Three-dimensional T1-weighted images were acquired with five MRI scanners and reconstructed with vendor-derived distortion correction. Some were reconstructed without correction to generate heavily distorted image-sets. All MR image-sets were corrected with the Brainlab algorithm relative to the computed tomography acquisition. CIRS Distortion Check software measured the distortion in each image-set. For all uncorrected and corrected image-sets, the control points that exceeded the 0.5-mm clinically relevant distortion threshold and the distortion maximum, mean, and standard deviation were recorded. Empirical cumulative distribution functions (eCDF) were plotted. Intraclass correlation coefficient (ICC) was calculated. The algorithm was evaluated with 10 brain metastases using Dice similarity coefficients (DSC).

RESULTS

The algorithm significantly reduced mean and standard deviation distortion in all image-sets. It reduced the maximum distortion in the heavily distorted image-sets from 2.072 to 1.059 mm and the control points with > 0.5-mm distortion fell from 50.2% to 4.0%. Before and especially after correction, the eCDFs of the four repeats were visually similar. ICC was 0.812 (excellent-good agreement). The algorithm increased the DSCs for all patients and image-sets.

CONCLUSION

The Brainlab algorithm significantly and reproducibly ameliorated MRI distortion, even with heavily distorted images. Thus, it increases the accuracy of cranial SRT lesion delineation. After further testing, this tool may be suitable for SRT of small lesions.

A strategy to prevent a temperature-induced MRI artifact in warm liquid phantoms due to convection currents

MRI phantom studies often fail to mimic the temperature of the human body, which can negatively impact accuracy. An artifact induced by increasing temperature in liquid phantoms was observed, presenting a significant challenge to temperature-controlled experiments. In this study we characterize and provide a solution to eliminate this temperature-induced MRI artifact. Low concentration (0.5-2.5 mM) agar phantoms were prepared. Utilizing a temperature-controlled phantom holder, T₁ - and T₂ -weighted structural images were acquired at 7 T along with quantitative B₀ , B₁ , T₁ , T₂ and ADC maps at both 25 and 37°C. Additionally, computer simulations were conducted to demonstrate the fluid flow and thermal flux patterns in water to provide an insight into the origins of the artifact. Evidence from computer simulation and quantitative MRI strongly suggest the artifact was caused by heat transfer in the form of natural convection leading to structured patterns of signal loss in MR images. The artifact was present up to agar concentrations of 1.5 mM (T₁ = 3068 ± 16 ms, T₂ = 1052 ± 20 ms, ADC = 2.29 ± 0.36 × 10^-3 mm² /s at 25°C; T₁ = 3928 ± 44 ms, T₂ = 1122 ± 24 ms, ADC = 2.64 ± 0.49 × 10^-3 mm² /s at 37°C), above which point increased sample viscosity no longer allows for convection currents, thereby eliminating the artifact. The methodology described in this work simplifies quantitative MR acquisition of liquid phantoms at physiological temperature by suppressing convection currents with relatively small changes to intrinsic MR parameters (T₁ increased by 1.4% and T₂ decreased by 17% for 1.5 mM agar at 25°C).

Evaluation of patient-specific MR distortion correction schemes for improved target localization accuracy in SRS

PURPOSE

This work aims at promoting target localization accuracy in cranial stereotactic radiosurgery (SRS) applications by focusing on the correction of sequence-dependent (also patient induced) magnetic resonance (MR) distortions at the lesion locations. A phantom-based quality assurance (QA) methodology was developed and implemented for the evaluation of three distortion correction techniques. The same approach was also adapted to cranial MR images used for SRS treatment planning purposes in single or multiple brain metastases cases.

METHODS

A three-dimensional (3D)-printed head phantom was filled with a 3D polymer gel dosimeter. Following treatment planning and dose delivery, volumes of radiation-induced polymerization served as hypothetical lesions, offering adequate MR contrast with respect to the surrounding unirradiated areas. T1-weighted (T1w) MR imaging was performed at 1.5 T using the clinical scanning protocol for SRS. Additional images were acquired to implement three distortion correction methods; the field mapping (FM), mean image (MI) and signal integration (SI) techniques. Reference lesion locations were calculated as the averaged centroid positions of each target identified in the forward and reverse read gradient polarity MRI scans. The same techniques and workflows were implemented for the correction of contrast-enhanced T1w MR images of 10 patients with a total of 27 brain metastases.

RESULTS

All methods employed in the phantom study diminished spatial distortion. Median and maximum distortion magnitude decreased from 0.7 mm (2.10 ppm) and 0.8 mm (2.36 ppm), respectively, to <0.2 mm (0.61 ppm) at all target locations, using any of the three techniques. Image quality of the corrected images was acceptable, while contrast-to-noise ratio slightly increased. Results of the patient study were in accordance with the findings of the phantom study. Residual distortion in corrected patient images was found to be <0.3 mm in the vast majority of targets. Overall, the MI approach appears to be the most efficient correction method from the three investigated.

CONCLUSIONS

In cranial SRS applications, patient-specific distortion correction at the target location(s) is feasible and effective, despite the expense of longer imaging time since additional MRI scan(s) need to be performed. A phantom-based QA methodology was developed and presented to reassure efficient implementation of correction techniques for sequence-dependent spatial distortion.