Abstract
A flexible-shaped ultrasonic array probe that can be used in a high magnetic field environment in an MRI gantry has been developed. Given that this probe can be fixed according to the shape of the skull’s surface, it is particularly applicable for imaging in the brain. To perform ultrasonic beamforming using a bent probe, it is necessary to measure the bent shape. Therefore, in this research, the curvature of the probe was estimated using MRI. A phantom with ellipse surface close to the shape of a skull was created using a 3D printer. The probe was arranged along the phantom surface to perform MRI and ultrasonic beamforming. The ultrasonic array transducer had 192 elements made from 1-3 composite piezoelectric materials with an element spacing of 0.3 mm. Eight MR position markers were attached to the probe in parallel in two rows of four each, with the array transducer in between. The delay time of each element in dynamic focusing for reception was calculated from its position estimated by the curvature of the probe. To evaluate the feasibility of this method, a B-mode image of 0.98 mm-diameter-thread targets placed in water was generated. The beam width at half maximum of the echo peak in the lateral direction from the thread target set up near the transmission focal point was calculated based on the B-mode image. It was found that the beam width of 1.32 mm in the proposed method, which was close to the thread diameter, whereas that was 4.38 mm in the conventional method that did not consider the bending of the probe. Consequently, the proposed beamforming technique is feasible for ultrasonic imaging through an arbitrary curved surface. Practical applications with a head phantom mimicking skull and cerebral tissue are expected.
Highlights
As typical clinical diagnosis, computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomographic (PET), and ultrasonic imaging are widely used [1]
An ultrasonic B-mode image with a conventional flat-type array probe is obtained by the calculation of the delay time of the ultrasonic wave reaching each element arranged on a straight line
The signal processing for the generation of the ultrasonic B-mode image in the proposed method was as follows; 1) (xi, yi) and were calculated from the curvature of the transducer approximated by a quadratic function using MRI; 2) the delay time τi′ of each element was calculated from Eq 3; 3) the acquisition data of the ultrasonic echo signals were delayed by τi′, and were summed; 4) a B-mode image was calculated with the logarithmic magnitude in a dynamic range of 60 dB
Summary
As typical clinical diagnosis, computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomographic (PET), and ultrasonic imaging are widely used [1]. V. Ozenna et al demonstrated that simultaneous measurement of thermal rise and displacement in the brain by a focus ultrasonic transcranial approach with a single-element transducer [3]. Ultrasonic B-mode images of the brain with blood flow representation were performed around 1990 [4] They propagated ultrasonic waves through an area bounded by the zygomatic process and the inferior temporal line by using a phased-array transducer with 48 elements. A flexible-type ultrasonic array probe with 192 elements was developed [5] It can be bent along curved surfaces such as the human body and skull. An ultrasonic B-mode image with a conventional flat-type array probe is obtained by the calculation of the delay time of the ultrasonic wave reaching each element arranged on a straight line. The effect of the correction on the B-mode image was described by comparison with the image without reference to the bending shape
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