A simple scheme to allow high-frequency ultrasound imaging systems to view objects at greater depth without compromising SNR is proposed in work from Dongguk University, Korea. This extended depth of field could allow greater use of high-frequency ultrasound for imaging and therapy. The team are now working on a prototype of the dual-concentric required by this method Conventional medical ultrasound imaging operates at frequencies of around 2 to 15 MHz. At these frequencies the ultrasound waves can penetrate deep into tissue, but the resolution of the resulting images is limited by the relatively long wavelength of the waves. High-frequency ultrasound imaging uses frequencies in excess of 20 MHz with the advantage of the smaller wavelengths allowing much better spatial resolution of the images in both the axial and lateral directions. However, the price of the improved resolution above 20 MHz is reduced penetration, due to increased attenuation of the waves in the tissue. Another way of describing this is as a reduction in the depth of field (DOF) – the range of distances or depths over which a system can maintain good focusing properties. This limits the application of high-frequency ultrasound imaging to near field targets such as in dermatology, ophthalmology, paediatrics and intravascular imaging. DOF is also important in therapeutic applications of ultrasound such as high-intensity focused ultrasound (HIFU), where sound waves are used to heat and destroy specific tissue. In this context, extending the DOF can also increase the area of the region that can be coagulated in each application, reducing the overall treatment time. One way to increase DOF in ultrasound imaging systems is to generate multiple focal points using an array transducer rather than a single element device; this is called multi-transmit focussing. The drawback with this method is that the use of array transducers incurs significant added complexity in a system's operation and fabrication. An axicon lens can also be used to improve DOF but at the cost of decreasing the intensity of the waves, especially for high-frequency ultrasound. The approach to extending DOF presented in the work from Dongguk University is the use of a phase-apodisation technique, previously used in optics, combined with a dual-concentric transducer. The results suggest that this approach can extend the DOF without compromising SNR when compared to a single element transducer. Cyst phantom simulation results showing the increased penetration allowed by the dual-concentric transducer/phase-apodisation approach (middle) over a single-element transducer (left) and finally the results with the compound mode that uses information from both (right) The scheme generates a bifocal zone in the axial direction by simultaneously activating the inner disc and outer ring-type element of the dual-concentric transducer using two input signals with 0° and 180° phases. The simulation results reported in this work show that the extension of the DOF may be due to the combination of the phase apodisation scheme and the dual-concentric transducer generating multiple foci along the axial direction. The relative simplicity of using a dual-concentric transducer and inverted phase inputs to improve DOF means that the hardware complexity will be less than with an array transducer multi-transit focussing approach, and the energy loss will be lower than that incurred through the use of an axicon lens. This approach does slightly reduce the spatial resolution compared to a single element transducer system, as well as introducing a low intensity zone at the focal depth. However, the Dongguk work includes a solution to both of these issues, as author Dr Jong Seob Jeong explained: “In order to compensate for abrupt intensity change between two lobes, a compound mode scheme is also proposed by synthesising data obtained from same and different phase excitations. “In the proposed method, the spatial resolution is slightly lower than with a conventional focusing scheme, however the improved brightness in the far field will help to monitor deeper targets. The proposed compound mode can be realised by the dual-concentric transducer which can be used for conventional imaging mode by applying same phase signal and phase inverted mode by applying phase apodisation.” The team are now working on a prototype dual-concentric transducer for experimental demonstration of their proposed technique. Their main challenge in this is proving to be sufficiently accurate alignment of the inner and outer transducer elements for a con-focal shaped aperture, but they believe this can be achieved with adjustments to their fabrication process.