# Real-Time HIFU Treatment Monitoring Using Pulse Inversion Ultrasonic Imaging
## 1 Introduction  
High-intensity focused ultrasound (HIFU) has attracted considerable attention as a non-invasive surgical tool; its applications range from cancer to brain disease treatment such as Alzheimer's disease [1; 2] and cosmetic treatment such as skin tightening [3; 4]. A pivotal element in HIFU surgery is an image guidance method for both pre-operative and intra-operative sessions. Magnetic resonance imaging (MRI) is typically used to identify a target lesion and determine a surgical plan pre-operatively. In an intra-operative session, either MRI or ultrasound is used for safe and accurate treatment guidance: estimation of temperature in treatment area [5; 6], HIFU focus targeting [7; 8] and motion tracking [9; 10], each with their own advantages and disadvantages. Compared to MRI, ultrasound imaging is the preferred guidance method for HIFU focus targeting and motion tracking because these functions require real-time imaging capability. Additionally, this method has the ability to monitor lesion change in real time in response to HIFU [11]. Because HIFU is a high-intensity signal, its echoes received by an imaging transducer have an amplitude higher than imaging echoes. The HIFU echoes are called HIFU interference signals in this paper. For this reason, the structural information in ultrasound images is inevitably buried in HIFU interference signals. This is so because an ultrasound imaging transducer receives not onlyimaging signals but also HIFU interference signals although the operating frequency of the imaging transducer is different from that of the HIFU transducer. To fully realize the benefits mentioned above, it is necessary to remove HIFU interference from imaging signals, so that it is possible to acquire the ultrasound images of the treatment area during HIFU surgery. To eliminate the HIFU interference pattern in ultrasound images, various methods have been proposed: HIFU excitation synchronized with ultrasound imaging sequence [7; 12], imaging signal acquisition during time periods for which HIFU is not delivered [13], coded excitation for imaging in conjunction with filtering [14; 15]. However, each method has different optimal condition of HIFU pulse length (i.e., either short or long pulse length) to achieve their best performance. Recently, we have proposed the HIFU interference cancellation method using pulse inversion, which does not have a constraint on HIFU pulse length theoretically [16; 17]. Similar to the general pulse-inversion method used in ultrasound tissue harmonic imaging [18], two HIFU pulses with phase \(0^{\circ}\) and \(180^{\circ}\) are transmitted in the previously proposed method. Since two sets of echoes received by an ultrasound imaging transducer contain both HIFU interference and imaging signals for one image scanline, the fundamental and odd-harmonic components of HIFU interference can be eliminated after adding the two sets of echoes; only imaging signals remain after band-pass filtering if the spectrum of the imaging signals is located near the frequency of the fundamental component or the odd-harmonic components of the HIFU interference. However, this method has a limitation on the spectral bandwidth for an imaging signal. For example, only 2-MHz bandwidth is available for imaging if the center frequency of HIFU is 1 MHz. Additionally, this method requires additional implementation cost, especially for array-based HIFU systems, because electrical pulse generators should be able to generate two electrical signals of \(0^{\circ}\) and \(180^{\circ}\) phases with no jitter between the two HIFU signals. To overcome the limitations, in this paper, we propose another approach to HIFU interference rejection using pulse inversion. Unlike the previous method, the proposed method employs the pulse inversion technique for imaging signals. For one image scanline, an imaging transducer fires ultrasound twice in \(0^{\circ}\) and \(180^{\circ}\) phases while HIFUs of the same phase are transmitted in synchronization with the ultrasound transmission for imaging.