The ultrasound brain helmet: new transducers and volume registration for in vivo simultaneous multi-transducer 3-D transcranial imaging

Abstract

Because stroke remains an important and time-sensitive health concern in developed nations, we present a system capable of fusing 3-D transcranial ultrasound volumes acquired from two sides of the head. This system uses custom sparse array transducers built on flexible multilayer circuits that can be positioned for simultaneous imaging through both temporal acoustic windows, allowing for potential registration of multiple real-time 3-D scans of cerebral vasculature. We examine hardware considerations for new matrix arrays-transducer design and interconnects-in this application. Specifically, it is proposed that SNR may be increased by reducing the length of probe cables. This claim is evaluated as part of the presented system through simulation, experimental data, and in vivo imaging. Ultimately, gains in SNR of 7 dB are realized by replacing a standard probe cable with a much shorter flex interconnect; higher gains may be possible using ribbon-based probe cables. In vivo images are presented, showing cerebral arteries with and without the use of microbubble contrast agent; they have been registered and fused using a simple algorithm which maximizes normalized cross-correlation.

Figures

Fig. 1

Probe placement for proposed transcranial imaging system using three 2-D arrays.

Fig. 2

Volume interrogated during a typical transtemporal 3-D ultra-sound examination. Coronal and transverse plane views are displayed simultaneously. A steerable Doppler beam is shown (blue in the online version).

Fig. 3

(a) Designed aperture (gray elements only transmit, white elements transmit and receive) and (b) resultant on-axis beamplot simulated using Field II. Simulations of the aperture steered to (c) +16° and (d) +32° in both azimuth and elevation at 2.5 MHz and a 7-cm focus.

Fig. 4

Photograph of an array after completion of dicing.

Fig. 5

Illustrative stack of transducer fabrication (not to scale). PZT is bonded to the polyimide flex circuit and diced, then two-sided LCP is bonded to the PZT. A lossy backing is bonded to the back of the flex (not shown).

Fig. 6

(a) schematic and (b) photograph of the completed system for in vivo scanning.

Fig. 7

(a) Single-element pulse echo-waveform from aluminum block reflector. (b) A 128-channel beamformed reflection from target at 7-cm focus.

Fig. 8

Real-time display view of contrast-enhanced flow in middle cerebral arteries (MCA), internal carotid arteries (ICA), and posterior cerebral arteries (PCA) using described system. Directional information of flow has been discarded. Labels have been added to indicate anatomy as well as the sectors displaying the subject's right coronal, right transverse, left coronal, and left transverse imaging planes.

Fig. 9

(a) Ultrasound rendering in the coronal plane and (b) representative magnetic resonance angiogram (MRA) (not the same subject) showing the area under ultrasound examination. (c) Paired ultrasound rendering in the transverse plane and (d) dissection image indicating anatomy in vessels of the Circle of Willis. ICA = internal carotid artery, MCA = middle cerebral artery, PCA = posterior cerebral artery. Original MRA image (b) produced by Ofir Glazer, Biomedical Engineering Department, Tel Aviv University, Israel. Reproduced with permission of the author. Original photograph (d) produced by Prof. John a. Beal, Department of Cellular Biology and Anatomy, Louisiana State Health Sciences Center, Shreveport, LA. Reproduced with permission of the author.

Fig. 10

(a) The same magnetic resonance angiogram of Fig. 9 is used to indicate the field of view for the registered, (b) rendered ultrasound data acquired without microbubble contrast agent. (c) an enlargement of the color flow data. Blue indicates data acquired by the right transducer; red indicates data acquired by the left transducer. (a) is reproduced with permission of the author, Ofir Glazer, Department of Biomedical Engineering, Tel Aviv University, Israel.

Fig. 11

Automatically registered rendering of posterior cerebral arteries from simultaneous transtemporal scans along with manually registered (yellow arrow) transforaminal scan. Vessels are paired with a dissection image (inset), with blue arrows indicating common structures. No microbubble contrast agent was used. Dissection image reproduced with permission of the author, Prof. John A. Beal.