Ultrasound-assisted convection-enhanced delivery to the brain in vivo with a novel transducer cannula assembly

Laboratory investigation

Restricted access

Object

In convection-enhanced delivery (CED), drugs are infused locally into tissue through a cannula inserted into the brain parenchyma to enhance drug penetration over diffusion strategies. The purpose of this study was to demonstrate the feasibility of ultrasound-assisted CED (UCED) in the rodent brain in vivo using a novel, low-profile transducer cannula assembly (TCA) and portable, pocket-sized ultrasound system.

Methods

Forty Sprague-Dawley rats (350–450 g) were divided into 2 equal groups (Groups 1 and 2). Each group was divided again into 4 subgroups (n = 5 in each). The caudate of each rodent brain was infused with 0.25 wt% Evans blue dye (EBD) in phosphate-buffered saline at 2 different infusion rates of 0.25 μl/minute (Group 1), and 0.5 μl/minute (Group 2). The infusion rates were increased slowly over 10 minutes from 0.05 to 0.25 μl/minute (Group 1) and from 0.1 to 0.5 μl/minute (Group 2). The final flow rate was maintained for 20 minutes. Rodents in the 4 control subgroups were infused using the TCA without ultrasound and without and with microbubbles added to the infusate (CED and CED + MB, respectively). Rodents in the 4 UCED subgroups were infused without and with microbubbles added to the infusate (UCED and UCED + MB) using the TCA with continuous-wave 1.34-MHz low-intensity ultrasound at a total acoustic power of 0.11 ± 0.005 W and peak spatial intensity at the cannula tip of 49.7 mW/cm2. An additional 4 Sprague-Dawley rats (350–450 g) received UCED at 4 different and higher ultrasound intensities at the cannula tip ranging from 62.0 to 155.0 mW/cm2 for 30 minutes. The 3D infusion distribution was reconstructed using MATLAB analysis. Tissue damage and morphological changes to the brain were assessed using H & E.

Results

The application of ultrasound during infusion (UCED and UCED + MB) improved the volumetric distribution of EBD in the brain by a factor of 2.24 to 3.25 when there were no microbubbles in the infusate and by a factor of 1.16 to 1.70 when microbubbles were added to the infusate (p < 0.001). On gross and histological examination, no damage to the brain tissue was found for any acoustic exposure applied to the brain.

Conclusions

The TCA and ultrasound device show promise to improve the distribution of infused compounds during CED. The results suggest further studies are required to optimize infusion and acoustic parameters for small compounds and for larger molecular weight compounds that are representative of promising antitumor agents. In addition, safe levels of ultrasound exposure in chronic experiments must be determined for practical clinical evaluation of UCED. Extension of these experiments to larger animal models is warranted to demonstrate efficacy of this technique.

Abbreviations used in this paper:AP = anteroposterior; CED = convection-enhanced delivery; CNC = computer numeric controlled; EBD = Evans blue dye; MB = microbubbles; MI = mechanical index; MOSFET = metal oxide semiconductor field-effect transistor; OCT = optimal cutting temperature; TCA = transducer cannula assembly; UCED = ultrasound-assisted CED.

Article Information

Address correspondence to: George K. Lewis Jr., Ph.D., ZetrOZ LLC, 421 Aurora Street, Ithaca, New York 14850. email: george@cornellbme.com.

Please include this information when citing this paper: published online September 21, 2012; DOI: 10.3171/2012.7.JNS11144.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Construction of the transducer cannula assembly. I: Machine the PZT-4 into a disk with center hole. II and III: Connect a brass tube to front face of ceramic using solder. IV: Place the ceramic in a watertight PVC/aluminum housing with sterotactic guide arm and connect ground and hot leads to the transducer through the guide arm. V and VI: Attach the guide cannula and infusion cannula to the transducer and secure to the proper height with epoxy. VII: Actual finished device. coax = coaxial.

  • View in gallery

    Animal experimental setup for using the TCA in rodent brain. The rodent is secured with ear bars in a stereotactic frame, and a small craniotomy is performed over the left hemisphere. The TCA is guided 5.5 mm deep into the caudate of the rodent brain.

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    Electrical impedance of TCA. Resonance occurs at 1.18 MHz with 380-Ω impedance. The phase angle (not shown) is approximately 0× at resonance. Parallel resonance occurs at 3.1 MHz.

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    Brain sections from the 4 subgroups of Group 2 after 30 minutes of Evans blue infusions at 0.5 μl/minute with a 30-gauge cannula. A and B: Convection-enhanced delivery (A) and CED with microbubbles (CED + MB) provide similar infusion profiles for the rodents in each group. C: Ultrasound-assisted CED delivers EBD further into the brain and more diffusely spread across the caudate. D: Ultrasound-assisted CED with microbubbles (UCED + MB) shows further EBD penetration than CED and CED + MB, but is more localized in the rodent caudate than UCED, which spreads EBD out of the caudate region. Backflow of EBD along the needle track into the white matter tract of the corpus callosum is reduced with UCED and UCED + MB as compared with controls.

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    Three-dimensional infusion reconstructions of brain sections from the 4 subgroups of Group 2 shown in Fig. 4. The cannula is in the plane of the figure; the TCA is positioned at the top of each figure. A: Convection-enhanced delivery. B: Convection-enhanced delivery with microbubbles. C: Ultrasound-assisted CED. D: Ultrasoundassisted CED with microbubbles.

  • View in gallery

    Analysis of total EBD volume distribution in the rodent brain with subgroup standard error bars. Ultrasound-assisted CED and UCED + MB increased EBD volume distribution by factors of 2.24 and 1.37 in the left hemisphere and 2.44 and 1.70 in the left caudate, respectively, as compared with CED and CED + MB in Group 1 animals receiving 0.25 μl/minute infusions. In the animals receiving 0.5 μl/minute infusions (Group 2), UCED and UCED + MB increased EBD volume distribution by factors of 2.96 and 1.16 in the left hemisphere and 3.25 and 1.54 in the left caudate, respectively. *The differences in volume distributions in the left hemisphere and left caudate subgroups of Group 1 and Group 2 are statistically significant with independent means p < 0.05 and p < 0.001.

  • View in gallery

    Group 1 (0.25 μl/minute infusion) analysis of EBD distribution profile in the rodent caudate as a function of the AP distance in the region ± 4 mm from the infusion site. The black line represents the average area of EBD at the given position. The shaded region represents the standard deviation of EBD area (n = 5).

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    Group 2 (0.5 μl/minute infusion) analysis of EBD distribution profile in the rodent caudate as a function of the AP distance in the region ± 4 mm from the infusion site. The black line represents the average area of EBD at the given position. The shaded region represents the standard deviation of EBD area (n = 5).

  • View in gallery

    The area of EBD in rodent caudate for each slice plotted as a function of the square of the AP distance from the infusion site for Group 1 and Group 2. Each solid colored line represents the mean area for each treatment at the given position. A dotted line segment with a slope magnitude of π is drawn for reference. Data that fall on a line with slope of π or −π indicate regions where the infusion of EBD is locally isotropic. Deviations from the slope indicate an anisotropic volume distribution.

  • View in gallery

    An H & E stain of rodent brains (10-μm coronal slices) in the cannula insertion path for the 40 rodents that received CED, CED + MB, UCED, and UCED + MB. Arrows point from the ×2.5 to ×20 magnification of histological results found in the cortex and caudate of the rodent brains. Mild parenchymal disruption, edema, and hemorrhage around the needle track and injection site are equivalent for all specimens.

  • View in gallery

    An H & E stain of rodent brains (10-μm coronal slices) in the cannula insertion path for the four rodents that received UCED of increased ultrasonic intensity. Arrows point from the ×2.5 to ×20 magnification of histological results found in the cortex and caudate of the rodent brains. Mild parenchymal disruption, edema, and hemorrhage around the needle track and injection site are equivalent for all specimens.

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