Single shot MR tagging to quantify local tissue deformation during MRI-guided needle interventions: a feasibility study.

PURPOSE: In MRI-guided needle interventions, such as biopsies and brachytherapy, tissue deformation caused by needle movement may result in localization errors and thus hamper the outcome of the procedure. Monitoring the local tissue deformation provides the ability to compensate for it, e.g., by increasing the needle insertion depth. Fast MR scans are useful to track the needle, but cannot be used to quantify local tissue deformation, in case anatomical landmarks are missing. Artificial landmarks can be created by MR tagging. This method provides a spatial saturation pattern (tag) in the tissue. Deformation of this pattern reflects the tissue motion between tag creation and tag imaging. As the needle movement is nonperiodic, k-space cannot be acquired with a multishot approach, like is usually done for cardiac imaging. Hence, a single shot MR tagging sequence is needed, which entails tag creation, needle movement and tag acquisition. In this study, the feasibility of single shot MR tagging for MRI-guided needle interventions in phantom and volunteer experiments is shown. METHODS: Four different experiments were performed on a 1.5 T MR scanner: the first to quantify translations, the second to quantify rotations, the third to mimic a needle intervention, and the fourth to investigate the tag persistence in a volunteer. The tag pattern is created by a 1331 composite pulse. A balanced steady state free precession sequence is used for imaging. To minimize undesired changes in contrast or sharpness of the tag pattern, we chose a relatively small flip angle and a short imaging time in all experiments. In the volunteer experiments, we modified the sequence to also be able to inspect the influence of the used k-space sampling profile and the flip angle on the temporal persistence of image contrast and tag pattern. In all scans, head or surface coils were used for signal reception. RESULTS: In all experiments, the tag pattern was clearly visible and could be used to quantify the local tissue deformation caused by (needle) movement. Strong correlations between the actual and measured (angular) phantom motions were obtained. In the needle intervention experiment, the tag lines were perfectly horizontal when there was no needle movement. With needle movement, local tissue displacements up to 5 mm were observed. Volunteer's anatomy could be discriminated, despite the tag pattern. The tag pattern in the prostate, for example, could still be read in all tagging images acquired 2 s after creating the tag pattern. With optimized scan parameters the tag persistence was even longer. The best image tag contrast was obtained using a large flip angle and the profile order low-high, although the image was slightly blurred. CONCLUSIONS: This study demonstrates that single shot MR tagging can be used to quantify tissue deformation caused by needle movement. The in-vivo tag persistence is sufficient to enable the application of the tagging sequence during MRI-guided needle interventions in patients.

New method to monitor RF safety in MRI-guided interventions based on RF induced image artefacts.

PURPOSE: Serious tissue heating may occur at the tips of elongated metallic structures used in MRI-guided interventions, such as vascular guidewires, catheters, biopsy needles, and brachytherapy needles. This heating is due to resonating electromagnetic radiofrequency (RF) waves along the structure. Since it is hard to predict the exact length at which resonance occurs under in vivo conditions, there is a need for methods to monitor this resonance behavior. In this study, the authors propose a method based on the RF induced image artefacts and demonstrate its applicability in two phantom experiments. METHODS: The authors developed an analytical model that describes the RF induced image artefacts as a function of the induced current in an elongated metallic structure placed parallel to the static magnetic field. It describes the total RF field as a sum of the RF fields produced by the transmit coil of the MR scanner and by the elongated metallic structure. Several spoiled gradient echo images with different nominal flip angle settings were acquired to map the B1+ field, which is a quantitative measure for the RF distortion around the structure. From this map, the current was extracted by fitting the analytical model. To investigate the sensitivity of our method we performed two phantom experiments with different setup parameters: One that mimics a brachytherapy needle insertion and one that resembles a guidewire intervention. In the first experiment, a short needle was placed centrally in the MR bore to ensure that the induced currents would be small. In the second experiment, a longer wire was placed in an off-center position to mimic a worst case scenario for the patient. In both experiments, a Luxtron (Santa Clara, CA) fiberoptic temperature sensor was positioned at the structure tip to record the temperature. RESULTS: In the first experiment, no significant temperature increases were measured, while the RF image artefacts and the induced currents in the needle increased with the applied insertion depth. The maximum induced current in the needle was 44 mA. Furthermore, a standing wave pattern became clearly visible for larger insertion depths. In the second experiment, significant temperature increases up to 2.4 degrees C in 1 min were recorded during the image acquisitions. The maximum current value was 1.4 A. In both experiments, a proper estimation of the current in the metallic structure could be made using our analytical model. CONCLUSIONS: The authors have developed a method to quantitatively determine the induced current in an elongated metallic structure from its RF distortion. This creates a powerful and sensitive method to investigate the resonant behavior of RF waves along elongated metallic structures used for MRI-guided interventions, for example, to monitor the RF safety or to inspect the influence of coating on the resonance length. Principally, it can be applied under in vivo conditions and for noncylindrical metallic structures such as hip implants by taking their geometry into account.

Feasibility of adequate dose coverage in permanent prostate brachytherapy using divergent needle insertion methods.

BACKGROUND AND PURPOSE: The purpose of this study is to investigate the feasibility of adequate dose coverage in permanent prostate brachytherapy using divergent needle insertion methods. These methods can be useful in magnetic resonance imaging (MRI) guided needle insertion techniques to avoid pubic arch interference. METHODS AND MATERIALS: MRI data were collected from 10 patients with T1-T2 prostate cancer. An inverse planning algorithm based on simulated annealing was used to optimize the dose distribution for three needle insertion methods: divergent needles with a single rotation point, divergent needles with a double rotation point, and current parallel needle insertion method. The dose constraints were based on our clinical criteria and the recent ESTRO/EAU/EORTC recommendations. RESULTS: If the planning target volume (PTV) surrounded the prostate and only intraprostatic seeds were allowed, the mean PTV volume that received 100% of the prescribed dose (V(100)) was 99% for all needle insertion methods. If the PTV was increased to the prostate with a 3mm margin, the mean PTV V(100) equalled 94%, 95%, and 94% for the single rotation point, double rotation point and current parallel needle insertion method, respectively. If in the latter case the tips of the seeds were placed 3mm outside the apex and base of the prostate, the mean PTV V(100) was 96% for all needle insertion methods. CONCLUSION: This planning study shows that it is feasible to generate an adequate dose coverage using divergent needle insertion methods.

A novel rectal applicator for contact radiotherapy with HDR 192Ir sources.

PURPOSE: Dose escalation to rectal tumors leads to higher complete response rates and may thereby enable omission of surgery. Important advantages of endoluminal boosting techniques include the possibility to apply a more selective/localized boost than using external beam radiotherapy. A novel brachytherapy (BT) rectal applicator with lateral shielding was designed to be used with a rectoscope for eye-guided positioning to deliver a dose distribution similar to the one of contact x-ray radiotherapy devices, using commonly available high-dose-rate 192Ir BT sources. METHODS AND MATERIALS: A cylindrical multichannel BT applicator with lateral shielding was designed by Monte Carlo modeling, validated experimentally with film dosimetry and compared with results found in the literature for the Papillon 50 (P50) contact x-ray radiotherapy device regarding rectoscope dimensions, radiation beam shape, dose fall-off, and treatment time. RESULTS: The multichannel applicator designed is able to deliver 30 Gy under 13 min with a 20350 U (5 Ci) source. The use of multiple channels and lateral shielding provide a uniform circular treatment surface with 22 mm in diameter. The resulting dose fall-off is slightly steeper (maximum difference of 5%) than the one generated by the P50 device with the 22 mm applicator. CONCLUSIONS: A novel multichannel rectal applicator for contact radiotherapy with high-dose-rate 192Ir sources that can be integrated with commercially available treatment planning systems was designed to produce a dose distribution similar to the one obtained by the P50 device.

A novel system for commissioning brachytherapy applicators: example of a ring applicator.

A novel system was developed to improve commissioning and quality assurance of brachytherapy applicators used in high dose rate (HDR). It employs an imaging panel to create reference images and to measure dwell times and dwell positions. As an example: two ring applicators of the same model were evaluated. An applicator was placed on the surface of an imaging panel and a HDR 192Ir source was positioned in an imaging channel above the panel to generate an image of the applicator, using the gamma photons of the brachytherapy source. The applicator projection image was overlaid with the images acquired by capturing the gamma photons emitted by the source dwelling inside the applicator. We verified 0.1, 0.2, 0.5 and 1.0 cm interdwell distances for different offsets, applicator inclinations and transfer tube curvatures. The data analysis was performed using in-house developed software capable of processing the data in real time, defining catheters and creating movies recording the irradiation procedure. One applicator showed up to 0.3 cm difference from the expected position for a specific dwell position. The problem appeared intermittently. The standard deviations of the remaining dwell positions (40 measurements) were less than 0.05 cm. The second ring applicator had a similar reproducibility with absolute coordinate differences from expected values ranging from -0.10 up to 0.18 cm. The curvature of the transfer tube can lead to differences larger than 0.1 cm whilst the inclination of the applicator showed a negligible effect. The proposed method allows the verification of all steps of the irradiation, providing accurate information about dwell positions and dwell times. It allows the verification of small interdwell positions (⩽0.1 cm) and reduces measurement time. In addition, no additional radiation source is necessary since the HDR 192Ir source is used to generate an image of the applicator.

Online pretreatment verification of high-dose rate brachytherapy using an imaging panel.

Brachytherapy is employed to treat a wide variety of cancers. However, an accurate treatment verification method is currently not available. This study describes a pre-treatment verification system that uses an imaging panel (IP) to verify important aspects of the treatment plan. A detailed modelling of the IP was only possible with an extensive calibration performed using a robotic arm. Irradiations were performed with a high dose rate (HDR) 192Ir source within a water phantom. An empirical fit was applied to measure the distance between the source and the detector so 3D Cartesian coordinates of the dwell positions can be obtained using a single panel. The IP acquires 7.14 fps to verify the dwell times, dwell positions and air kerma strength (Sk). A gynecological applicator was used to create a treatment plan that was registered with a CT image of the water phantom used during the experiments for verification purposes. Errors (shifts, exchanged connections and wrong dwell times) were simulated to verify the proposed verification system. Cartesian source positions (panel measurement plane) have a standard deviation of about 0.02 cm. The measured distance between the source and the panel (z-coordinate) have a standard deviation up to 0.16 cm and maximum absolute error of ≈0.6 cm if the signal is close to sensitive limit of the panel. The average response of the panel is very linear with Sk. Therefore, Sk measurements can be performed with relatively small errors. The measured dwell times show a maximum error of 0.2 s which is consistent with the acquisition rate of the panel. All simulated errors were clearly identified by the proposed system. The use of IPs is not common in brachytherapy, however, it provides considerable advantages. It was demonstrated that the IP can accurately measure Sk, dwell times and dwell positions.

MRI-guided robotic system for transperineal prostate interventions: proof of principle.

In this study, we demonstrate the proof of principle of the University Medical Center Utrecht (UMCU) robot dedicated to magnetic resonance imaging (MRI)-guided interventions in patients. The UMCU robot consists of polymers and non-ferromagnetic materials. For transperineal prostate interventions, it can be placed between the patient's legs inside a closed bore 1.5T MR scanner. The robot can manually be translated and rotated resulting in five degrees of freedom. It contains a pneumatically driven tapping device to automatically insert a needle stepwise into the prostate using a controller unit outside the scanning room. To define the target positions and to verify the needle insertion point and the needle trajectory, a high-resolution 3D balanced steady state free precession (bSSFP) scan that provides a T2/T1-weighted contrast is acquired. During the needle insertion fast 2D bSSFP images are generated to track the needle on-line. When the target position is reached, the radiation oncologist manually places a fiducial gold marker (small seed) at this location. In total two needle trajectories are used to place all markers. Afterwards, a high-resolution 3D bSSFP scan is acquired to visualize the fiducial gold markers. Four fiducial gold markers were placed transperineally into the prostate of a patient with a clinical stage T3 prostate cancer. In the generated scans, it was possible to discriminate the patient's anatomy, the needle and the markers. All markers were delivered inside the prostate. The procedure time was 1.5 h. This study proves that MRI-guided needle placement and seed delivery in the prostate with the UMCU robot are feasible.