Measurement of the ultrasound backscatter signal from three seed types as a function of incidence angle: application to permanent prostate brachytherapy

Feasibility of vibro-acoustography with a quasi-2D ultrasound array transducer for detection and localizing of permanent prostate brachytherapy seeds: a pilot ex vivo study

PURPOSE

Effective permanent prostate brachytherapy (PPB) requires precise placement of radioactive seeds in and around the prostate. The impetus for this research is to examine a new ultrasound-based imaging modality, vibro-acoustography (VA), which may serve to provide a high rate of PPB seed detection while also effecting enhanced prostate imaging. The authors investigate the ability of VA, implemented on a clinical ultrasound (US) scanner and equipped with a quasi-2D (Q2D) array US transducer, to detect and localize PPB seeds in excised prostate specimens.

METHODS

Nonradioactive brachytherapy seeds were implanted into four excised cadaver prostates. A clinical US scanner equipped with a Q2D array US transducer was customized to acquire both US and C-scan VA images at various depths. The VA images were then used to detect and localize the implanted seeds in prostate tissue. To validate the VA results, computed tomography (CT) images of the same tissue samples were obtained to serve as the reference by which to evaluate the performance of VA in PPB seed detection.

RESULTS

The results indicate that VA is capable of accurately identifying the presence and distribution of PPB seeds with a high imaging contrast. Moreover, a large ratio of the PPB seeds implanted into prostate tissue samples could be detected through acquired VA images. Using CT-based seed identification as the standard, VA was capable of detecting 74%-92% of the implanted seeds. Additionally, the angular independency of VA in detecting PPB seeds was demonstrated through a well-controlled phantom experiment.

CONCLUSIONS

Q2DVA detected a substantial portion of the seeds by using a 2D array US transducer in excised prostate tissue specimens. While VA has inherent advantages associated with conventional US imaging, it has the additional advantage of permitting detection of PPB seeds independent of their orientation. These results suggest the potential of VA as a method for PPB imaging that ultimately may allow US-based real-time intraoperative dosimetry.

Photoacoustic imaging of prostate brachytherapy seeds

Brachytherapy seed therapy is an increasingly common way to treat prostate cancer through localized radiation. The current standard of care relies on transrectal ultrasound (TRUS) for imaging guidance during the seed placement procedure. As visualization of individual metallic seeds tends to be difficult or inaccurate under TRUS guidance, guide needles are generally tracked to infer seed placement. In an effort to improve seed visualization and placement accuracy, the use of photoacoustic (PA) imaging, which is highly sensitive to metallic objects in soft tissue, was investigated for this clinical application. The PA imaging properties of bare (i.e., embedded in pure gelatin) and tissue-embedded (at depths of up to 13 mm) seeds were investigated with a multi-wavelength (750 to 1090 nm) PA imaging technique. Results indicate that, much like ultrasonic (US) imaging, an angular dependence (i.e., seed orientation relative to imaging transducer) of the PA signal exists. Despite this shortcoming, however, PA imaging offers improved contrast, over US imaging, of a seed in prostate tissue if sufficient local fluence is achieved. Additionally, although the PA signal of a bare seed is greatest for lower laser wavelengths (e.g., 750 nm), the scattering that results from tissue tends to favor the use of higher wavelengths (e.g., 1064 nm, which is the primary wavelength of Nd:YAG lasers) when the seed is located in tissue. A combined PA and US imaging approach (i.e., PAUS imaging) shows strong potential to visualize both the seed and the surrounding anatomical environment of the prostate during brachytherapy seed placement procedures.

Real-time image-based B-mode ultrasound image simulation of needles using tensor-product interpolation

In this paper, we propose an interpolation-based method for simulating rigid needles in B-mode ultrasound images in real time. We parameterize the needle B-mode image as a function of needle position and orientation. We collect needle images under various spatial configurations in a water-tank using a needle guidance robot. Then we use multidimensional tensor-product interpolation to simulate images of needles with arbitrary poses and positions using collected images. After further processing, the interpolated needle and seed images are superimposed on top of phantom or tissue image backgrounds. The similarity between the simulated and the real images is measured using a correlation metric. A comparison is also performed with in vivo images obtained during prostate brachytherapy. Our results, carried out for both the convex (transverse plane) and linear (sagittal/para-sagittal plane) arrays of a trans-rectal transducer indicate that our interpolation method produces good results while requiring modest computing resources. The needle simulation method we present can be extended to the simulation of ultrasound images of other wire-like objects. In particular, we have shown that the proposed approach can be used to simulate brachytherapy seeds.

Detection of brachytherapy seeds using 3-D transrectal ultrasound

Detection of brachytherapy seeds plays a key role in dosimetry for prostate brachytherapy. However, seed localization using B-mode transrectal ultrasound (TRUS) still remains a challenge for prostate brachytherapy, mainly due to the small size of brachytherapy seeds in the relatively low-quality B-mode TRUS images. In this paper, we propose a new solution for brachytherapy seed detection using 3-D ultrasound. A 3-D reflected power image is computed from ultrasound RF signals, instead of conventional B-mode images. Then, implanted seeds are segmented in 3-D local search spaces that are determined by a priori knowledge, e.g., needle entry points and seed placements. Needle insertion tracks are also detected locally by the Hough transform. Experimental results show that the proposed solution works well for seed localization in a prostate phantom implanted according to a realistic treatment plan with 136 seeds from 26 needles.

Acoustic backscattering enhancements resulting from the interaction of an obliquely incident plane wave with an infinite cylinder

BACKGROUND AND OBJECTIVE

The analysis of the acoustic backscattering enhancements from tilted cylinders is of particular importance in determining some of the (visco)elastic properties of the cylinder, and/or its surrounding fluid in ultrasonic non-destructive evaluation (NDE) and imaging (NDI) applications. Previous related investigations on an aluminum cylinder limited to incidence angles varying from 0 degrees to 40 degrees , revealed the existence of an anomalous "pseudo-Rayleigh" mode (above the critical Rayleigh angle) identified as the rigid-body translational dipole (n=1) mode. The objective here is to provide a complete investigation on the backscattering enhancements for incidence angles larger than 40 degrees for various elastic and viscoelastic cylinder materials.

METHOD

Using the partial-wave series solution for the linear scattering by an infinite circular cylinder, the acoustic backscattering from isotropic elastic and viscoelastic (polymer-type) cylinders excited by an obliquely incident plane acoustic wave is investigated. Total and resonance backscattering form functions are calculated for several elastic and viscoelastic cylinder materials immersed in water versus the angle of incidence 0 degrees alpha < 90 degrees . The "pure" resonance peaks are isolated by subtracting a rigid background from the total form function, so the associated resonance modes are properly identified.

RESULTS AND CONCLUSION

The plots of the partial-wave series reveal acoustic backscattering enhancements (not shown in previous investigations) generally occurring at ka less, similar 0.1 at a critical angle alpha(c) bounded by the longitudinal and shear waves coupling angles theta(L)=sin(-1)(c/c(L)) and theta(S)=sin(-1)(c/c(S)) such that theta(L) < alpha(c) < theta(S) (where c(L) and c(S) are the phase velocities of the longitudinal and shear waves inside the elastic cylinder, and c is the speed of sound in the surrounding medium). It is shown here that the backscattering enhancements with a critical angle theta(L) < alpha(c) < theta(S) result from the excitation of the monopole (n=0) resonance mode. Moreover, additional acoustic backscattering enhancements still occur in the range 1 less, similar ka less similar 6 even though the angle of tilt is greater than the Rayleigh wave coupling angle theta(R)=sin(-1)(c/c(R)) (where c(R) is the Rayleigh wave velocity in an elastic half-space). The resonance scattering theory shows that such additional enhancements are associated with the excitation of a dipole (n=1) resonance mode which may result from the interference of meridional and/or helical waves propagating along the cylinder's surface. It is therefore essential to consider tilt angles ranging from normal to end-on incidence for a complete analysis of the backscattering by elastic and viscoelastic cylinders.

Seed localization and TRUS-fluoroscopy fusion for intraoperative prostate brachytherapy dosimetry

OBJECTIVE

To develop and evaluate an integrated approach to intra-operative dosimetry for permanent prostate brachytherapy (PPB) by combining a fluoroscopy-based seed localization routine with a transrectal ultrasound (TRUS)-to-fluoroscopy fusion technique.

MATERIALS AND METHODS

Three-dimensional seed coordinates are reconstructed based on the two-dimensional seed locations identified from three fluoroscopic images acquired at different angles. A seed-based registration approach was examined in both simulation and phantom studies to register the seed locations identified from the fluoroscopic images to the TRUS images. Dose parameters were then evaluated and compared to CT-based dosimetry from a patient dataset.

RESULTS

Less than 0.2% error in the D90 value was observed using the TRUS-fluoroscopy image-fusion-based method relative to the CT-based post-implantation dosimetry. In the phantom study, an average distance of 3 mm was observed between the seeds identified from TRUS and the reconstructed seeds at registration. Isodose contours were displayed superimposed on the TRUS images.

CONCLUSIONS

Promising results were observed in this preliminary study of a TRUS-fluoroscopy fusion-based brachytherapy dosimetry analysis method, suggesting that the method is highly sensitive and calculates clinically relevant dosimetry, including the prostate D90. Further validation of the method is required for eventual clinical application.

Vibro-acoustography imaging of permanent prostate brachytherapy seeds in an excised human prostate--preliminary results and technical feasibility

OBJECTIVE

The objective in this work is to investigate the feasibility of using a new imaging tool called vibro-acoustography (VA) as a means of permanent prostate brachytherapy (PPB) seed localization to facilitate post-implant dosimetry (PID).

METHODS AND MATERIALS

Twelve OncoSeed (standard) and eleven EchoSeed (echogenic) dummy seeds were implanted in a human cadaver prostate. Seventeen seeds remained after radical retropubic prostatectomy. VA imaging was conducted on the prostate that was cast in a gel phantom and placed in a tank of degassed water. 2-D magnitude and phase VA image slices were obtained at different depths within the prostate showing location and orientation of the seeds.

RESULTS

VA demonstrates that twelve of seventeen (71%) seeds implanted were visible in the VA image, and the remainder were obscured by intra-prostatic calcifications. Moreover, it is shown here that VA is capable of imaging and locating PPB seeds within the prostate independent of seed orientation, and the resulting images are speckle free.

CONCLUSION

The results presented in this research show that VA allows seed detection within a human prostate regardless of their orientation, as well as imaging intra-prostatic calcifications.

In vitro comparative study of vibro-acoustography versus pulse-echo ultrasound in imaging permanent prostate brachytherapy seeds

BACKGROUND

Permanent prostate brachytherapy (PPB) is a common treatment for early stage prostate cancer. While the modern approach using trans-rectal ultrasound guidance has demonstrated excellent outcome, the efficacy of PPB depends on achieving complete radiation dose coverage of the prostate by obtaining a proper radiation source (seed) distribution. Currently, brachytherapy seed placement is guided by trans-rectal ultrasound imaging and fluoroscopy. A significant percentage of seeds are not detected by trans-rectal ultrasound because certain seed orientations are invisible making accurate intra-operative feedback of radiation dosimetry very difficult, if not impossible. Therefore, intra-operative correction of suboptimal seed distributions cannot easily be done with current methods. Vibro-acoustography (VA) is an imaging modality that is capable of imaging solids at any orientation, and the resulting images are speckle free.

OBJECTIVE AND METHODS

The purpose of this study is to compare the capabilities of VA and pulse-echo ultrasound in imaging PPB seeds at various angles and show the sensitivity of detection to seed orientation. In the VA experiment, two intersecting ultrasound beams driven at f(1)=3.00 MHz and f(2)=3.020 MHz respectively were focused on the seeds attached to a latex membrane while the amplitude of the acoustic emission produced at the difference frequency 20 kHz was detected by a low frequency hydrophone.

RESULTS

Finite element simulations and results of experiments conducted under well-controlled conditions in a water tank on a series of seeds indicate that the seeds can be detected at any orientation with VA, whereas pulse-echo ultrasound is very sensitive to the seed orientation.

CONCLUSION

It is concluded that vibro-acoustography is superior to pulse-echo ultrasound for detection of PPB seeds.

Towards detecting surgical clips in 3D ultrasound for target localization in radiation therapy: a study on tissue phantoms

A study is performed to measure the visibility of small surgical clips in 3D ultrasound volumes of a phantom as the first step towards using such clips as fiducial markers in radiation therapy. Visibility is calculated as the contrast to noise ratio of the echo from the clip to the background. The appearance of a resonance tail is also calculated using a moment analysis. Contrast is found to be high and mostly independent of material and size of the clip. All clips are visible, but the length of the tail is found to be dependent on clip orientation to the ultrasound beam. The consistent visibility of the clips suggests they are suitable as fiducial markers in ultrasound and the dependency of the resonance tail on orientation provides an opportunity to distinguish clips from other specular reflectors.

Dosimetry accuracy as a function of seed localization uncertainty in permanent prostate brachytherapy: increased seed number correlates with less variability in prostate dosimetry

The variation of permanent prostate brachytherapy dosimetry as a function of seed localization uncertainty was investigated for I-125 implants with seed activities commonly employed in contemporary practice. Post-implant imaging and radiation dosimetry data from nine patients who underwent permanent prostate brachytherapy served as the source of clinical data for this simulation study. Gaussian noise with standard deviations ranging from 0.5 to 10 mm was applied to the seed coordinates for each patient dataset and 1000 simulations were performed at each noise level. Dose parameters, including D90, were computed for each case and compared with the actual dosimetry data. A total of 81 000 complete sets of post-brachytherapy dose volume statistics were computed. The results demonstrated that less than 5% deviation of prostate D90 can be expected when the seed localization uncertainty is 2 mm, whereas a seed localization uncertainty of 10 mm yielded an average decrease in D90 of 33 Gy. The mean normalized decrement in the prostate V100 was 10% at 5 mm uncertainty. Implants with greater seed number and larger prostate volume correlated with less sensitivity of D90 and V100 to seed localization uncertainty. Estimated target volume dose parameters tended to decrease with increasing seed localization uncertainty. The bladder V100 varied more significantly both in mean and standard deviation as compared to the urethra V100. A larger number of implanted seeds also correlated to less sensitivity of the bladder V100 to seed localization uncertainty. In contrast, the deviation of urethra V100 did not correlate with the number of implanted seeds or prostate volume.

Examination of dosimetry accuracy as a function of seed detection rate in permanent prostate brachytherapy

The variation of permanent prostate brachytherapy dosimetry as a function of seed detection rates was investigated for I125 implants with seed activities commonly employed in contemporary practice. Post-implant imaging and radiation dosimetry data from nine patients who underwent PPB served as the basis of this simulation study. One-thousand random configurations of detected seeds were generated for each patient dataset using various seed detection levels from 30% to 99%. Dose parameters, including D90, were computed for each configuration and compared with the actual dosimetry data. A total of 108 000 complete sets of post-PPB dose volume statistics were computed. The results demonstrated that although the average D90 differed from the true value by less than 5% when 70% or more seeds were identified, the D90 of an individual case could deviate up to 13%. The 95% confidence interval (CI) of estimated D90 values differ by less than 5% from the actual value when 95% or more seeds are detected, or approximately a 7 Gy difference in the D90 value for a prescription dose of 144 Gy. Estimated target volume dose parameters tended to decrease with reduced seed detection rates. The most variable dose parameter was the prostate V100 in absolute scale while the urethral V100 was most variable in a relative sense. Based on this comprehensive simulation study, it is suggested that 95% or more seeds need to be localized in order to provide an accurate estimation of dose parameters for contemporary iodine 125 permanent prostate brachytherapy.

Treatment planning consideration for prostate implants with the new linear RadioCoil 103Pd brachytherapy source

Recently, various linear source models, for example, 103Pd RadioCoil, have been introduced to overcome the shortcomings of traditional "seed" type interstitial prostate brachytherapy implants, such as migration and clumping of the seeds. However, the existing prostate treatment-planning systems have not been updated to perform dose calculation for implants with linear sources greater than 1.0 cm in length. In these investigations, two new models are developed for 3D dose calculation for a prostate implant with linear brachytherapy sources using the commercially available treatment-planning systems. The proposed models are referred to as the linear-segmented source (LSS) model and the point-segmented source (PSS) model. The calculated dose distributions obtained by these models for a single linear source have been validated by their comparison with the Monte Carlo-simulated data. Moreover, these models were used to calculate the dose distributions for a multilinear source prostate implant, and the results were compared to "seed" type implants. The results of these investigations show that the LSS model better approximates the linear sources than the PSS model. Moreover, these models have shown a better approximation of the dose distribution from a linear source for 0.5 cm source segments as compared to 1.0 cm source segments. However, for the points close to the longitudinal axis of the source located outside the region bounded by the active length, both models show differences of approximately +/-15%. These deficiencies are attributed to the limitations of the TG43 formalism for elongated sources.

Impact of selection of post-implant technique on dosimetry parameters for permanent prostate implants

PURPOSE

To investigate the variability of prostate implant quality indices between three different methods of calculating the post-implant dose distribution.

METHODS AND MATERIALS

In a study of 9 permanent prostate implant patients, post-implant dosimetry was carried out using three methods of identifying seed positions within the prostate volume: (1) prostate volumes defined by transrectal ultrasound (TRUS) immediately following implant were registered with shift-film defined seed positions, (2) seeds were identified directly from the post-implant TRUS images, and (3) CT was used to define seed positions and prostate volumes from images acquired at 41-65 days post-implant. For each method, the volume of prostate receiving 90%, 100%, and 150% of the prescribed dose (V90, V100, V150) and the dose delivered to 90% of the prostate volume (D90) were calculated.

RESULTS

Post-implant TRUS volumes were within 15% of the preimplant TRUS volumes in 8 of the 9 patients investigated. The post-implant CT volume was within 15% of the preimplant (TRUS) volume in only 3 of the 9 cases. The value of the dosimetry parameters was dependent on the method used and varied by 5-25% for V90, 5-30% for V100, 42-134% for V150, and 9-60% for D90. No simple relationship was found between change in volume and the resultant change in dosimetry parameter. Differences in dosimetry parameters due to source localization uncertainties was found to be small (< or = 10% for V100) when comparing methods (1) and (2).

CONCLUSIONS

There are many uncertainties in the calculation of parameters that are commonly used to describe the quality of a permanent prostate implant. Differences in the parameters calculated were most likely a result of a combination of factors including uncertainties in delineating the prostate with different imaging modalities, differences in source identification techniques, and intraobserver variability.

Prostate volume measurement by transrectal ultrasound and computed tomography before and after permanent prostate brachytherapy

PURPOSE

To quantify prostate volume (pvol) changes with transrectal ultrasound (TRUS) immediately after permanent prostate brachytherapy (PPB) and to correlate these changes with postimplant computed tomography (CT) volumetrics. To provide data relevant to evaluating the potential of TRUS-based image fusion for intraoperative dosimetry.

METHODS AND MATERIALS

Between July 2000 and January 2003, 177 patients underwent (125)I PPB monotherapy at our institution, and 165 patients provided research authorization. A total of 136 patients (82%) completed 4 imaging studies: planning TRUS, intraoperative pre- and postimplant TRUS, and CT.

RESULTS

Mean planning TRUS pvol was 38.7 +/- 11.7 cc standard deviation (SD), 95% confidence interval (CI) (36.7, 40.7). Mean intraoperative TRUS pvol preimplant was 37.1 +/- 11.7 cc SD, 95% CI (35.1, 39.0), and postimplant was 44.5 +/- 15.1 cc SD, 95% CI (42.0, 47.1). The mean ratio of postimplant:preimplant intraoperative TRUS pvols was 1.2 +/- 0.2 SD, 95% CI (1.18, 1.24), and the difference in mean values was 7.5 cc (p < 0.0001). CT performed within 1 day revealed a mean pvol of 47.9 +/- 15.7 cc SD, 95% CI (45.2, 50.5). The mean volumetric ratio of CT to postimplant TRUS pvol was 1.13 +/- 0.36, 95% CI (1.07-1.19).

CONCLUSIONS

Whereas mean preimplant step-section TRUS pvol measurements are similar, postimplant TRUS and CT measurements have greater variability that depend on initial pvol. CT-based pvol measurements determined a mean of 10.6 hours after implant were more likely to be identical to those of immediate postimplant TRUS in prostates >33 cc. These data are relevant for establishing accuracy in image-fusion based approaches being investigated for real-time intraoperative PPB dosimetry.