Passive marker tracking via phase-only cross correlation (POCC) for MR-guided needle interventions: Initial in vivo experience

Image-guided prostate biopsy robots: A review

At present, the incidence of prostate cancer (PCa) in men is increasing year by year. So, the early diagnosis of PCa is of great significance. Transrectal ultrasonography (TRUS)-guided biopsy is a common method for diagnosing PCa. The biopsy process is performed manually by urologists but the diagnostic rate is only 20%-30% and its reliability and accuracy can no longer meet clinical needs. The image-guided prostate biopsy robot has the advantages of a high degree of automation, does not rely on the skills and experience of operators, reduces the work intensity and operation time of urologists and so on. Capable of delivering biopsy needles to pre-defined biopsy locations with minimal needle placement errors, it makes up for the shortcomings of traditional free-hand biopsy and improves the reliability and accuracy of biopsy. The integration of medical imaging technology and the robotic system is an important means for accurate tumor location, biopsy puncture path planning and visualization. This paper mainly reviews image-guided prostate biopsy robots. According to the existing literature, guidance modalities are divided into magnetic resonance imaging (MRI), ultrasound (US) and fusion image. First, the robot structure research by different guided methods is the main line and the actuators and material research of these guided modalities is the auxiliary line to introduce and compare. Second, the robot image-guided localization technology is discussed. Finally, the image-guided prostate biopsy robot is summarized and suggestions for future development are provided.

Evaluation of cine imaging during multileaf collimator and gantry motion for real-time magnetic resonance guided radiation therapy

Purpose

Real‐time magnetic resonance guided radiation therapy (MRgRT) uses 2D cine imaging for target tracking. This work evaluates the percent image uniformity (PIU) and spatial integrity of cine images in the presence of multileaf collimator (MLC) and gantry motion in order to simulate sliding window and volumetric modulated arc therapy (VMAT) conditions.

Methods

Percent image uniformity and spatial integrity of cine images were measured (1) during MLC motion, (2) as a function of static gantry position, and (3) during gantry rotation. PIU was calculated according to the ACR MRI Quality Control Manual. Spatial integrity was evaluated by measuring the geometric distortion of 16 measured marker positions (10 cm or 15.225 cm from isocenter).

Results

The PIU of cine images did not vary by more than 1% from static linac conditions during MLC motion and did not vary by more than 3% during gantry rotation. Banding artifacts were present during gantry rotation. The geometric distortion in the cine images was less than 0.88 mm for all points measured throughout MLC motion. For all static gantry positions, the geometric distortion was less than 0.88 mm at 10 cm from isocenter and less than 1.4 mm at 15.225 cm from isocenter. During gantry rotation, the geometric distortion remained less than 0.92 mm at 10 cm from isocenter and less than 1.60 mm at 15.225 cm from isocenter.

Conclusion

During MLC motion, cine images maintained adequate PIU, and the geometric distortion of points within 15.225 cm from isocenter was less than the 1 mm threshold necessary for real‐time target tracking and gating. During gantry rotation, PIU was negatively affected by banding artifacts, and spatial integrity was only maintained within 10 cm from isocenter. Future work should investigate the effects imaging artifacts have on real‐time target tracking during MRgRT.

Passive needle guide tracking with radial acquisition and phase-only cross-correlation

PURPOSE

Acceleration of a passive tracking sequence based on phase-only cross-correlation (POCC) using radial undersampling.

METHODS

The phase-only cross-correlation (POCC) algorithm allows passive tracking of interventional instruments in real-time. In a POCC sequence, two cross-sectional images of a needle guide with a positive MR contrast are continuously acquired from which the instrument trajectory is calculated. Conventional Cartesian imaging for tracking is very time consuming; here, a higher temporal resolution is achieved using a highly undersampled radial acquisition together with a modified POCC algorithm that incorporates the point-spread-function. Targeting and needle insertion is performed in two phantom experiments with 16 fiducial targets, each using 4 and 16 radial projections for passive tracking. Additionally, targeting of eight deep lying basivertebral veins in the lumbar spines is performed for in vivo proof-of-application with four radial projections for needle guide tracking.

RESULTS

The radially undersampled POCC sequence yielded in the phantom experiments a lateral targeting accuracy of 1.1 ± 0.4 mm and 1.0 ± 0.5 mm for 16 and 4 radial projections, respectively, without any statistically significant difference. In the in vivo application, a mean targeting duration of 62 ± 13 s was measured.

CONCLUSION

Radial undersampling can drastically reduce the acquisition time for passive tracking in a POCC sequences for MR-guided needle interventions without compromising the targeting accuracy.

GantryMate: A Modular MR-Compatible Assistance System for MR-Guided Needle Interventions

Percutaneous minimally invasive interventions are difficult to perform in closed-bore high-field magnetic resonance systems owing to the limited space between magnet and patient. To enable magnetic resonance–guided needle interventions, we combine a small, patient-mounted assistance system with a real-time instrument tracking sequence based on a phase-only cross-correlation algorithm for marker detection. The assistance system uses 2 movable plates to align an external passive marker with the anatomical target structure. The targeting accuracy is measured in phantom experiments, yielding a precision of 1.7 ± 1.0 mm for target depths up to 38 ± 13 mm. In in vivo experiments, the possibility to track and target static and moving structures is demonstrated.

Optimizing accuracy and precision of micro-coil localization in active-MR tracking

OBJECTIVE

To examine whether a centroid peak detection algorithm and micro-transmit tracking improve the accuracy and precision of active-MR tracking when combined with previously published strategies of Hadamard Multiplexing and Phase-field Dithering.

MATERIALS AND METHODS

The dipole magnetic field of a solenoid tracking coil was modeled and MR spin excitation using both a uniform body coil and the tracking coil was simulated for 5329 orientations of the solenoid coil with respect to B0. A lumenless micro-coil was built onto a rotation platform and MR-tracking accuracy and precision were experimentally assessed for 576 orientations within a 1.5-T MRI scanner. Peak identification strategies (i.e. maximum pixel detection and the centroid pixel method) and transmit modes (body transmit and micro-transmit tracking) were employed and localization accuracy was assessed for each orientation in both simulation and experimentation.

RESULTS

The simultaneous use of the centroid pixel method, micro-transmit tracking, Phase-field Dithering, and Hadamard Multiplexing resulted in high MR tracking accuracy and precision: 0.52±0.41 mm and 0.34 mm respectively. Furthermore, all four methods combined offered a tracking error less than the size of the micro-coil, despite the lack of a signal source within the micro-coil.

CONCLUSIONS

Micro-transmit tracking and the centroid pixel method improve MR tracking accuracy and precision when combined with Phase-field Dithering and Hadamard Multiplexing compared to using Phase-field Dithering and Hadamard Multiplexing alone.

Position and orientation measurement of susceptibility markers using spectrally selective spin-echo projections

PURPOSE

To develop an accurate technique for simultaneously measuring the position and orientation of interventional devices using a projection-based spectrally selective refocusing pulse sequence.

METHODS

Projections along physical axes using spectrally selective excitation were acquired to track a catheter. A 9F passive tracking device capable of generating controllable susceptibility artifacts using susceptibility materials (titanium and graphite) was attached to the catheter. A library of projections for different orientations of the device with respect to the main magnetic field were simulated offline. Cross-correlations with these templates were computed to determine the orientation and position of the device. A phantom study was performed to evaluate the accuracy of the tracking technique. The tracking technique was also evaluated in vivo in the carotid artery of a pig.

RESULTS

Simultaneous and accurate measurement of position and orientation of the tracking device was obtained in the phantom and in vivo studies with reasonable temporal resolution. For the phantom study, the average of absolute errors in the Z-, Y-, and X-axes are 0.37, 0.76, and 0.85 mm, respectively. The mean absolute error and standard deviation of orientation measurement are 1.5 and 1.1 degrees, respectively.

CONCLUSION

This positioning technique, in conjunction with a controllable tracking device, can provide accurate tracking of interventional devices in MR-guided interventions. Magn Reson Med 76:1563-1573, 2016. © 2015 International Society for Magnetic Resonance in Medicine.

Autonomous real-time interventional scan plane control with a 3-D shape-sensing needle

This study demonstrates real-time scan plane control dependent on three-dimensional needle bending, as measured from magnetic resonance imaging (MRI)-compatible optical strain sensors. A biopsy needle with embedded fiber Bragg grating (FBG) sensors to measure surface strains is used to estimate its full 3-D shape and control the imaging plane of an MR scanner in real-time, based on the needle's estimated profile. The needle and scanner coordinate frames are registered to each other via miniature radio-frequency (RF) tracking coils, and the scan planes autonomously track the needle as it is deflected, keeping its tip in view. A 3-D needle annotation is superimposed over MR-images presented in a 3-D environment with the scanner's frame of reference. Scan planes calculated based on the FBG sensors successfully follow the tip of the needle. Experiments using the FBG sensors and RF coils to track the needle shape and location in real-time had an average root mean square error of 4.2 mm when comparing the estimated shape to the needle profile as seen in high resolution MR images. This positional variance is less than the image artifact caused by the needle in high resolution SPGR (spoiled gradient recalled) images. Optical fiber strain sensors can estimate a needle's profile in real-time and be used for MRI scan plane control to potentially enable faster and more accurate physician response.

Automated real-time needle-guide tracking for fast 3-T MR-guided transrectal prostate biopsy: a feasibility study

PURPOSE

To assess the feasibility of automatic needle-guide tracking by using a real-time phase-only cross correlation ( POCC phase-only cross correlation ) algorithm-based sequence for transrectal 3-T in-bore magnetic resonance (MR)-guided prostate biopsies.

MATERIALS AND METHODS

This study was approved by the ethics review board, and written informed consent was obtained from all patients. Eleven patients with a prostate-specific antigen level of at least 4 ng/mL (4 μg/L) and at least one transrectal ultrasonography-guided biopsy session with negative findings were enrolled. Regions suspicious for cancer were identified on 3-T multiparametric MR images. During a subsequent MR-guided biopsy, the regions suspicious for cancer were reidentified and targeted by using the POCC phase-only cross correlation -based tracking sequence. Besides testing a general technical feasibility of the biopsy procedure by using the POCC phase-only cross correlation -based tracking sequence, the procedure times were measured, and a pathologic analysis of the biopsy cores was performed.

RESULTS

Thirty-eight core samples were obtained from 25 regions suspicious for cancer. It was technically feasible to perform the POCC phase-only cross correlation -based biopsies in all regions suspicious for cancer in each patient, with adequate biopsy samples obtained with each biopsy attempt. The median size of the region suspicious for cancer was 8 mm (range, 4-13 mm). In each region suspicious for cancer (median number per patient, two; range, 1-4), a median of one core sample per region was obtained (range, 1-3). The median time for guidance per target was 1.5 minutes (range, 0.7-5 minutes). Nineteen of 38 core biopsy samples contained cancer.

CONCLUSION

This study shows that it is feasible to perform transrectal 3-T MR-guided biopsies by using a POCC phase-only cross correlation algorithm-based real-time tracking sequence.

An MR-compatible stereoscopic in-room 3D display for MR-guided interventions

BACKGROUND AND METHODS

A commercial three-dimensional (3D) monitor was modified for use inside the scanner room to provide stereoscopic real-time visualization during magnetic resonance (MR)-guided interventions, and tested in a catheter-tracking phantom experiment at 1.5 T. Brightness, uniformity, radio frequency (RF) emissions and MR image interferences were measured.

RESULTS AND DISCUSSION

Due to modifications, the center luminance of the 3D monitor was reduced by 14%, and the addition of a Faraday shield further reduced the remaining luminance by 31%. RF emissions could be effectively shielded; only a minor signal-to-noise ratio (SNR) decrease of 4.6% was observed during imaging. During the tracking experiment, the 3D orientation of the catheter and vessel structures in the phantom could be visualized stereoscopically.