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Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC San Francisco Electronic Theses and Dissertations (2016)

Purpose

By measuring the discrepancy of distance in electromagnetic tracking (EM) versus ultrasound images to calculate an error due to speed of sound (SOS) variances, this work establishes correctional values for inputs into post processing algorithms that will provide a correction based on B-Mode ultrasound images and consequently radiation dose plans in brachytherapy procedures.

Background

The speed of sound (SOS) within the soft tissue category is approximated to be 1540 m/s [1]. This fixed value is used for the SOS despite actual ranges in vivo varying considerably with one study measuring values of 1450 to 1613m/s in the field of view (FOV) [2]. The current state of technology doesn’t include a reasonable method for a patient-specific recalibration. Furthermore, inaccurate assignment of acoustic velocity may result in speed displacement and dispersion artifacts, especially at organ boundaries [2]. Brachytherapy for prostate cancer utilizes a transrectal ultrasound (TRUS) probe , which can provide 300-500 micron spatial resolution images for anatomical reference during needle implantation; however, the local composition and configuration of this soft tissue anatomy may exhibit a range of SOSs within the imaging field [3]. Images acquired with a SOS based on 1540 m/s and used in conjunction with stereotactic external coordinate systems can cause image distortion and displacement errors of several millimeters. In combination with the steep 1/r2 dose falloff around radioactive seeds, this can lead to erroneous dose delivery [4, 5]. Previous studies have outlined accuracy of 3-6 mm, leaving ample space for future robotics to overtake the manual process of seed implantation [6].

Method & Materials

The Aurora EM tracking system (NDI Waterloo, Ontario, Canada) generates a 500mm x 500mm x 500mm local electromagnetic (EM) field which can detect the spatial location and orientation of sensors placed within. Philips (Amsterdam, The Netherlands) has developed a brachytherapy needle with an embedded sensor that produces a signal in the EM coordinate system.

Results

Using custom-built phantoms with SOS ranging between 1430 and 1530 m/s, EM tracking resolution was verified to <1 mm precision while US localization resulted in up to 3 mm displacements from physical measurement. We present a model of correction to mitigate error in needle trajectories due to SOS variances and using these values a correctional system will be demonstrated to show the effects of changes in dosimetry due to patient specific SOS variances in the prostate. Results show SOS measurements to be within 1% of measured using a water tank system.

Conclusion

Previous studies have demonstrated that correct speed through tissue can be corrected, but may involve ionizing radiation or physical reference for these corrections [2,7]. The technique proposed will provide a new input of data, derived from the EM modality to approximate a more accurate SOS and result in an evaluation of accuracy of current clinically used systems.

    Cover page: Evaluation of needle displacement errors in ultraosund-based brachytherapy using electromagnetic (EM) tracking
    • Article
    • Peer Reviewed
    UC San Francisco Previously Published Works (2019)
    Older HIV-infected patients are at risk for both HIV-associated neurocognitive disorder (HAND) and Alzheimer's disease. We investigated neuroimaging and neuropsychological performance of 61 virally suppressed older adults with HAND (mean (SD) age 64.3 (3.9) years), 53 demographically matched individuals with mild cognitive impairment of the Alzheimer's type (MCI-AD; 65.0 [4.8]), and 89 healthy controls (65.0 [4.3]) cross-sectionally and over 20 months. At the baseline, both disease groups exhibited lower volumes in multiple cortical and subcortical regions compared with controls. Hippocampal volume differentiated MCI-AD from HAND. Cognitively, MCI-AD performed worse on memory and language compared with HAND. Adjusted longitudinal models revealed greater diffuse brain atrophy in MCI-AD compared with controls, whereas HAND showed greater atrophy in frontal gray matter and cerebellum compared with controls. Comparing HAND with MCI-AD showed similar atrophy rates in all brain regions explored, with no significant findings. MCI-AD exhibited more pronounced language decline compared with HAND. These findings reveal the need for further work on unique cognitive phenotypes and neuroimaging signatures of HAND compared with early AD, providing preliminary clinical insight for differential diagnosis of age-related brain dysfunction in geriatric neuroHIV.
      Cover page: Longitudinal brain atrophy patterns and neuropsychological performance in older adults with HIV-associated neurocognitive disorder compared with early Alzheimer's disease
      • Article
      • Peer Reviewed
      UC San Francisco Previously Published Works (2017)

      Background

      Current HIV treatments are successful at suppressing plasma HIV RNA to undetectable levels for most adherent patients. Yet, emerging evidence suggests that viral suppression will inadequately control inflammation and mitigate risk for progressive brain injury. We sought to quantify differences in longitudinal brain atrophy rates among older virally suppressed HIV-infected participants compared with that of healthy aging participants.

      Methods

      We examined longitudinal structural brain magnetic resonance imaging atrophy rates using region of interest assessments and voxel-wise tensor-based morphometry in HIV-infected participants older than 60 years (n = 38) compared with age-matched HIV-uninfected healthy and cognitively normal controls (n = 24).

      Results

      The mean age of participants was 63 years, the mean estimated duration of infection was 21 years, and the median duration of documented viral suppression was 3.2 years. Average proximal and nadir CD4 counts were 550 and 166, respectively; 15/38 (39%) met criteria for HIV-associated neurocognitive disorder. In models adjusting for age and sex, HIV serostatus was associated with more rapid average annualized rates of atrophy in the cerebellum (0.42% vs. 0.02%, P = 0.016), caudate (0.74% vs. 0.03%, P = 0.012), frontal lobe (0.48% vs. 0.01%, P = 0.034), total cortical gray matter (0.65% vs. 0.16%, P = 0.027), brainstem (0.31% vs. 0.01%, P = 0.026), and pallidum (0.73% vs. 0.39%, P = 0.046). Among those with HIV, atrophy rates did not differ statistically by cognitive status.

      Conclusions

      Despite persistent control of plasma viremia, these older HIV-infected participants demonstrate more rapid progressive brain atrophy when compared with healthy aging. Either HIV or other factors that differ between older HIV-infected participants and healthy controls could be responsible for these differences.
        Cover page: Progressive Brain Atrophy Despite Persistent Viral Suppression in HIV Patients Older Than 60 Years
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