In Part I of this article, the definitions, prevalence, and clinical presentation of chronic total occlusions (CTOs) were reviewed, the histopathology of CTOs was examined, efforts to replicate human CTOs with experimental models were appraised, and the clinical relevance and rationale for CTO revascularization were evaluated.1 In Part II, we summarize the technical approach to and outcomes after percutaneous coronary intervention (PCI) of occluded coronary arteries, describe the novel devices and drugs approved and undergoing investigation for CTO recanalization, and conclude with practical perspectives on managing the patient with 1 or more chronic coronary occlusions.

### Patient Selection and Revascularization Strategies

PCI of CTOs constitutes as many as 20% of all angioplasty procedures at selected centers,2 although a rate of &10% is more typical,3–6 suggesting that CTO angioplasty is attempted in 50 000 to 100 000 patients per year in the United States. Many more CTOs are present for which PCI is never attempted, representing one of the most common causes for referral to bypass surgery rather than PCI.6–8 Furthermore, a large proportion of patients with CTOs are managed medically, the prognosis of whom may vary depending on the extent of viable myocardium and ischemia, concomitant atherosclerosis in other coronary and noncoronary vascular territories, and other comorbid conditions. The decision to attempt PCI of a CTO (versus continued medical therapy or surgical revascularization) requires an individualized risk/benefit analysis, encompassing clinical, angiographic, and technical considerations. Clinically, the patient’s age, symptom severity, associated comorbidities (eg, diabetes mellitus and chronic renal insufficiency), and overall functional status are major determinants of treatment strategy. Angiographically, the extent and complexity of coronary artery disease (eg, single-vessel versus multivessel disease, single versus multiple total occlusions, likelihood for complete revascularization), left ventricular function, and the presence and degree of valvular heart disease should be considered. The technical probability of achieving …

Percutaneous recanalization of chronically occluded coronary arteries: a consensus document: part II.

This paper, the first from a series of three papers on the application of coded excitation signals in medical ultrasound, discusses the basic principles and ultrasound-related problems of pulse compression. The concepts of signal modulation and matched filtering are given, and a simple model of attenuation relates the matched filter response with the ambiguity function, known from radar. Based on this analysis and the properties of the ambiguity function, the selection of coded waveforms suitable for ultrasound imaging is discussed. It is shown that linear frequency modulation (FM) signals have the best and most robust features for ultrasound imaging. Other coded signals such as nonlinear FM and binary complementary Golay codes also have been considered and characterized in terms of signal-to-noise ratio (SNR) and sensitivity to frequency shifts. Using the simulation program Field II, it is found that in the case of linear FM signals, a SNR improvement of 12 to 18 dB can be expected for large imaging depths in attenuating media, without any depth-dependent filter compensation. In contrast, nonlinear FM modulation and binary codes are shown to give a SNR improvement of only 4 to 9 dB when processed with a matched filter. Other issues, such as depth-dependent matched filtering and use of filters other than the matched filter (inverse and Wiener filters) also are addressed.

Use of modulated excitation signals in medical ultrasound. Part I: basic concepts and expected benefits

Resolution and penetration are primary criteria for clinical image quality. Conventionally, high bandwidth for resolution was achieved with a short pulse, which results in a tradeoff between resolution and penetration. Coded excitation extends the bounds of this tradeoff by increasing signal-to-noise ratio (SNR) through appropriate coding on transmit and decoding on receive. Although used for about 50 years in radar, coded excitation was successfully introduced into commercial ultrasound scanners only within the last 5 years. This delay is at least partly due to practical implementation issues particular to diagnostic ultrasound, which are the focus of this paper. After reviewing the basics of biphase and chirp coding, we present simulation results to quantify tradeoffs between penetration and resolution under frequency-dependent attenuation, dynamic focusing, and nonlinear propagation. Next, we compare chirp and Golay code performance with respect to image quality and system requirements, then we show clinical images that illustrate the current applications of coded excitation in B-mode, harmonic, and flow imaging.

Coded excitation for diagnostic ultrasound: a system developer's perspective

Capacitive micromachined ultrasonic transducers (CMUTs) have been subject to extensive research for the last two decades. Although they were initially developed for air-coupled applications, today their main application space is medical imaging and therapy. This paper first presents a brief description of CMUTs, their basic structure and operating principles. Our progression of developing several generations of fabrication processes is discussed with an emphasis on the advantages and disadvantages of each process. Monolithic and hybrid approaches for integrating CMUTs with supporting integrated circuits are surveyed. Several prototype transducer arrays with integrated front-end electronic circuits we developed and their use for 2D and 3D, anatomical and functional imaging, and ablative therapies are described. The presented results prove the CMUT as a micro-electro-mechanical systems technology for many medical diagnostic and therapeutic applications.

/pdf/capacitive-micromachined-ultrasonic-transducers-for-medical-1c8bymjvrh.pdf

Capacitive micromachined ultrasonic transducers for medical imaging and therapy

Many applications of ultrasound for sensing, actuation and imaging require miniaturized and low power transducers and transducer arrays integrated with electronic systems. Piezoelectric micromachined ultrasound transducers (PMUTs), diaphragm-like thin film flexural transducers typically formed on silicon substrates, are a potential solution for integrated transducer arrays. This paper presents an overview of the current development status of PMUTs and a discussion of their suitability for miniaturized and integrated devices. The thin film piezoelectric materials required to functionalize these devices are discussed, followed by the microfabrication techniques used to create PMUT elements and the constraints the fabrication imposes on device design. Approaches for electrical interconnection and integration with on-chip electronics are discussed. Electrical and acoustic measurements from fabricated PMUT arrays with up to 320 diaphragm elements are presented. The PMUTs are shown to be broadband devices with an operating frequency which is tunable by tailoring the lateral dimensions of the flexural membrane or the thicknesses of the constituent layers. Finally, the outlook for future development of PMUT technology and the potential applications made feasible by integrated PMUT devices are discussed.

/pdf/piezoelectric-micromachined-ultrasound-transducer-pmut-48hf8j9o1k.pdf

Piezoelectric Micromachined Ultrasound Transducer (PMUT) Arrays for Integrated Sensing, Actuation and Imaging

Intravascular ultrasound (IVUS) imaging systems using circumferential arrays mounted on cardiac catheter tips fire beams orthogonal to the principal axis of the catheter. The system produces high resolution cross-sectional images but must be guided by conventional angioscopy. A real-time forward-viewing array, integrated into the same catheter, could greatly reduce radiation exposure by decreasing angiographic guidance. Unfortunately, the mounting requirement of a catheter guide wire prohibits a full-disk imaging aperture. Given only an annulus of array elements, prior theoretical investigations have only considered a circular ring of point transceivers and focusing strategies using all elements in the highly dense array, both impractical assumptions. In this paper, we consider a practical array geometry and signal processing architecture for a forward-viewing IVUS system. Our specific design uses a total of 210 transceiver firings with synthetic reconstruction for a given 3-D image frame. Simulation results demonstrate this design can achieve side-lobes under -40 dB for on-axis situations and under -30 dB for steering to the edge of a 60/spl deg/ cone.

Optimizing the beam pattern of a forward-viewing ring-annular ultrasound array for intravascular imaging

Peak acoustic power limits the signal-to-noise ratio (SNR) of real-time ultrasound images. For most conventional scan formats, however, the average power is well below heating limits. This means the SNR can be significantly increased using coded excitation. A coded system transmits a broadband, temporally elongated excitation pulse with a finite time-bandwidth product. The received signal must be decoded to produce an imaging pulse with improved SNR resulting from the higher average power in the elongated excitation. Decoding can produce significant range side lobes, however, greatly reducing image quality. All practical coding designs, therefore, represent a trade-off between SNR gain and range side lobes. A specific coding scheme appropriate for synthetic aperture imaging is presented. A 14.5 dB SNR improvement with acceptable range side lobes is demonstrated on a forward-looking imaging system appropriate for intravascular applications.

Coded excitation for synthetic aperture ultrasound imaging

In this paper, a scheme called coded excitation with spectrum inversion (CEXSI) is presented. An established optimal binary code whose spectrum has no nulls and possesses the least variation is encoded as a burst for transmission. Using this optimal code, the decoding filter can be derived directly from its inverse spectrum. Various transmission techniques can be used to improve energy coupling within the system pass-band. We demonstrate its potential to achieve excellent decoding with very low (<80 dB) side-lobes. For a 2.6 /spl mu/s code, an array element with a center frequency of 10 MHz and fractional bandwidth of 38%, range side-lobes of about 40 dB have been achieved experimentally with little compromise in range resolution. The signal-to-noise ratio (SNR) improvement also has been characterized at about 14 dB. Along with simulations and experimental data, we present a formulation of the scheme, according to which CEXSI can be extended to improve SNR in sparse array imaging in general.

Coded EXcitation with spectrum inversion (CEXSI) for ultrasound array imaging

In this paper, we present some initial results from our proposed forward viewing ring-annular array for intravascular ultrasound imaging. We have investigated array design and image synthesis allowing simultaneous sideward imaging at 20 MHz and forward imaging at 10 MHz from the same array. This design is appropriate for over-the-wire delivery but limits the forward-looking aperture to a ring-annular array. In particular, a 1200 /spl mu/m diameter ring-annular array with 64 elements has been constructed. The measured forward- viewing radiation pattern of an individual element indicates an acceptance angle range of over 90/spl deg/. Currently, there are significant sensitivity and center frequency variations among the elements. Nevertheless, images of point targets are obtained as a proof-of-concept. Future improvements of the array will be discussed.

http://ieeexplore.ieee.org/iel5/58/25996/01159845.pdf

Forward-Viewing Ring-Annular Ultrasound Array for Intravascular Imaging

http://uix.sagepub.com/content/23/1/19.full.pdf

Yao Wang

Papers

Optimizing the beam pattern of a forward-viewing ring-annular ultrasound array for intravascular imaging

Coded excitation for synthetic aperture ultrasound imaging

Coded EXcitation with spectrum inversion (CEXSI) for ultrasound array imaging

Forward-Viewing Ring-Annular Ultrasound Array for Intravascular Imaging

Modified weighting method for forward-looking ring-annular arrays.