Original Contribution
Acoustic Accessibility Investigation for Ultrasound Mediated Treatment of Glycogen Storage Disease Type Ia Patients

https://doi.org/10.1016/j.ultrasmedbio.2011.06.004Get rights and content

Abstract

Glycogen storage disease type Ia (GSDIa) is caused by an inherited defect in the glucose-6-phosphatase gene. The recent advent of targeted ultrasound-mediated delivery (USMD) of plasmid DNA (pDNA) to the liver in conjunction with microbubbles may provide an alternative treatment option. This study focuses on determining the acoustically accessible liver volume in GSDIa patients using transducer models of various geometries with an image-based geometry-driven approach. Results show that transducers with longer focal lengths and smaller apertures (up to an f/number of 2) are able to access larger liver volumes in GSDIa patients while still being capable of delivering the required ultrasound dose in situ (2.5 MPa peak negative pressure at the focus). With sufficiently large acoustic windows and the ability to use glucose to easily assess efficacy, GSD appears to be a good model for testing USMD as proof of principle as a potential therapy for liver applications in general.

Introduction

Glycogen storage diseases are caused by various genetic defects that impair glycogen storage or use. Even though such diseases have relatively low incident rates in the general population (approximately 1 in 100,000 births in the United States) (Lei et al. 1994), patients, especially infants, are at high risk of life-threatening acute hypoglycemia, which can lead to seizures, coma, and even death. Glycogen storage disease type Ia (GSDIa), or von Gierke disease, is caused by a defective glucose-6-phosphatase (G6Pase) gene, which prevents the synthesis of the G6Pase enzyme (Lei et al., 1993, Shieh et al., 2002). Without the G6Pase enzyme, the liver is unable to hydrolyze glucose from glucose-6-phosphate, which is an intermediate from either metabolism of stored glycogen or gluconeogenesis. No cure for GSDIa is currently available, and strict dietary control has been the only method to manage the disease in the past several decades (Moses 2002). Although GSDIa is considered to be a rare disease compared with other prevalent health problems, attention has been drawn to this research area, especially because the quality of life of these patients is significantly reduced. In addition, it is an attractive model to study liver disease because glucose concentrations can be used and easily measured as a biomarker of enzyme activity.

Recent advances in gene therapy may offer a potential cure of GSDIa disease (Weinstein et al., 2010, Koeberl et al., 2008, Seip et al., 2010). It is based on introducing nondefective G6Pase genes to liver cells to restore the production of the glucose-6-phosphatase enzyme. Reliable delivery of this therapeutic gene to the target liver cells continues to be a challenge. Viral vectors, such as adeno-associated virus (AAV), have been shown to transport genes into the cells (Zingone et al. 2000; Chou et al. 2007), but this method suffers from the risk of an antiviral immune or inflammatory response. An alternative approach uses therapeutic ultrasound in combination with microbubbles to extravasate and transfect pDNA into cells via sonoporation and other noninvasive mechanisms (Shen et al., 2008, Hernot and Klibanov, 2008). In this approach, the plasmid and the microbubbles are co-injected into the bloodstream while focused ultrasound pulses are applied to the region being targeted (Phillips et al. 2010). It has been shown (Lin et al. 2010) that this approach increases the permeability of blood vessels and cell walls, which eventually leads to an enhanced uptake of the plasmid by the ultrasound-targeted cells. In addition, microbubbles are also used as contrast agents, enabling real-time planning and guidance for the procedure (Qin et al. 2009).

Before ultrasound can be applied to deliver pDNA to liver to possibly treat GSDIa, it is necessary to understand potential fundamental limitations of this technique. Because of the impedance mismatch between soft tissue and bone/air, ultrasound is unable to penetrate through tissue types such as the ribs and lungs. Liver, the target of this treatment, is partially hidden inside the rib cage, limiting the volume that would be accessible by the beam emanating from an ultrasound transducer. Therefore, knowledge of the acoustically accessible liver volume of GSDIa patients is a prerequisite needed to determine whether USMD techniques can be successfully applied to deliver pDNA into liver tissue. Several treatment planning strategies have been proposed for trans-rib ablation and hyperthermia using focused ultrasound: Civale et al. (2006) used a linear segmented transducer for rib-sparing thermal treatment; Liu et al. (2007) reported trans-rib ablation by turning off obstructed elements from a 2-D array transducer. In this paper, we propose a fast and efficient method for acoustic accessibility estimation and attempt to determine this information as a function of transducer parameters (i.e., aperture, focal depth). For this purpose, 3-D imaging datasets of five GSDIa patients (magnetic resonance imaging [MRI] or computed tomography [CT] scans) were analyzed. An algorithm was developed that calculates the total accessible liver volume (in percent) based on a given transducer geometry and patient dataset, using a geometric modeling approach. Various transducer geometries were investigated in the aim of finding optimal parameters to treat the maximum liver volume without the ultrasound transducer beam interfering with either bone or lung structures. The resulting transducer candidates were then further evaluated using acoustic field simulations to determine which of these would also be able to generate the required pressures at their focal spot for USMD, thus generating initial specifications for transducer and/or array designs suitable for delivering ultrasound in GSDIa patients.

Section snippets

Methods

A geometry-based approach, similar to the one described in McGough et al. (1996), combined with acoustic field simulations was developed to determine the liver acoustical accessibility for GSDIa patients. In this study, single-element focused transducers with various geometries (apertures from 30 to 100 mm and focal lengths from 60 to 140 mm) were evaluated for their acoustical accessibility to liver tissues. This approach provides a conservative but simple estimate of the accessible liver

Results

To quantify the impact of transducer focal length on the accessible liver volume, the average accessible liver volume percentage of GSDIa patients was plotted as a function of focal length for a given aperture (Fig. 4). The accessible liver volume gradually increases as the transducer focal length increases. This indicates that (in general and neglecting other considerations, such as transducer/patient coupling), transducers with longer focal lengths are able to reach more liver tissue,

Discussion

A large ultrasound window is advantageous for any ultrasound treatments, including USMD treatments, high-intensity focused ultrasound (HIFU) treatments, hyperthermia treatments, etc., because it provides larger, unobstructed access to the target region (Dubinsky et al., 2008, Zhu et al., 2009). This simplifies transducer positioning, treatment planning and treatment execution, while reducing treatment risks and increasing the probability of a better treatment outcome. In HIFU treatments, tumor

Conclusions

This study provided an estimate of one of the most basic parameters needed to determine whether ultrasound-mediated delivery treatments would be feasible for GSDIa patients—the acoustically accessible liver volume as a function of transducer geometry and acoustic parameters. A geometry-based approach was chosen to compute the acoustic accessible liver volume for various transducers, and the acoustic performance of suitable transducer candidates was verified via simulations. The algorithm used

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