Intracavity and High-Frequency (HF) Imaging

1. Chapter 8 of Diagnostic Ultrasound imaging and blood flow measurment H. Saberi Intracavity and High-Frequency (HF) Imaging

2. Introduction • Imaging • Transesophageal Cardiac Imaging • TRANSRECTAL AND TRANSVAGINAL IMAGING • ENDOLUMINAL IMAGING • INTRAVASCULAR IMAGING • HIGH-FREQUENCY IMAGING • ACOUSTIC MICROSCOPES

3. Conventional ultrasonic imaging systems typically use frequencies from 2 to 15 MHz. Lower frequencies have a larger depth of penetration but poorer resolution. To improve spatial resolution, one obvious strategy would be to increase the frequency. Introduction

6. The price to be paid is an increase in attenuation.

7. Intracavity imaging such as • Transesophagealcardial imaging, • transrectal, and transvaginal imaging and • Endoluminal imaging is a partial solution to achieving improvements in spatial resolution. • Imaging organs such as the heart, prostate, and uterus/ovary from the body surface does not usually allow the utilization of frequencies higher than 5 MHz because they are deep-lying organs; • Under these conditions, higher frequencies may be used because the probes are in closer contact with the organ of interest. Imaging

8. During scanning, the transesophageal probe is inserted into the esophagus and the tip is positioned against the wall of the esophagus under local anesthesia. • A majority of the probes are capable of bi-plane imaging, i.e., two orthogonal images are produced. • Transesophageal imaging of the heart yields better images of the whole heart. • Additional benefits are that transesophageal imaging allows continuous monitoring of the cardiac functions. Transesophageal Cardiac Imaging

10. Probes at frequencies higher than 5 MHz are available for most of imaging systems for insertion into the rectum or vagina for better imaging of the prostate and uterus/ovary. • A linear array or curved linear array is mounted at the side or at the tip of a probe. • A full bladder, which used to be recommended for transabdominal obstetrical imaging of a fetus, may now be replaced by transvaginal imaging. TRANSRECTAL AND TRANSVAGINAL IMAGING

12. Catheter-based imaging systems have also been used to image the gastrointestinal tract, including colon, esophagus, and stomach. A few manufacturers have developed specialized ultrasonic imaging systems to accomplish this by mounting ultrasonic transducers/arrays on the end of an endoscope. ENDOLUMINAL IMAGING

13. Imaging of the wall of blood vessels for the purpose of estimation of the degree of stenosis and characterization of atherosclerotic plaques has been pursued for many years with a variety of imaging modalities. • In the past, x-ray angiography has been the gold standard for assessing stenosis. • As a result, its role is being challenged by magnetic resonance imaging and ultrasound. INTRAVASCULAR IMAGING

14. Imaging options for characterizing plaque composition are quite limited. • X-ray angiogeraphy • Fiberopticangioscopy, • and transcutaneous ultrasound with injection of saline. • Intravascular ultrasound and • optical coherent tomography (OCT) • are possible alternatives to alleviating these problems. • Intravascular ultrasound scanners typically are operated in the frequency range from 20 to 60 MHz depending upon the imaging catheter used. INTRAVASCULAR IMAGING

15. Two different types of imaging catheters are on the market today. • A mechanically rotated transducer (top) and • An array wrapped around the circumference. • Also mounted on the catheter with the array are several integrated circuit chips that perform the functions of low noise broadband preamplification and multiplexing. • A synthetic imaging approach in which one element transmits and 14 elements receive is used to form the image. INTRAVASCULAR IMAGING

17. Also mounted on the catheter with the array are several integrated circuit chips that perform the functions of low noise broadband preamplification and multiplexing. A synthetic imaging approach in which one element transmits and 14 elements receive is used to form the image. INTRAVASCULAR IMAGING

20. Scanners operated at frequencies higher than 20 MHz have been developed for applications in • ophthalmology, • dermatology, and • small-animal imaging. • These devices, called ultrasonic backscatter microscopes or ultrasonic biomicroscopes (UBMs), typically obtain images by scanning a single-element ultrasonic transducer in a sector format or linearly. • The construction of a UBM is identical to that of a static B-mode scanner. HIGH-FREQUENCY IMAGING

21. Scanning can also be achieved by better utilizing the focus of the transducer by incrementally moving the transducer in the axial direction, called B–D (D stands for depth) mode scan. • A composite image is formed by combing the focused segments of multiple images acquired as the transducer is moved in the axial direction. • This mode of scanning improves lateral resolution by sacrificing frame rate. • Commercial UBMs can achieve a frame rate of 30/sec because the excursion range of the transducer is extremely small. HIGH-FREQUENCY IMAGING

22. Conventional dicing technology may be used to fabricate linear arrays up to 50 MHz. • For fabricating linear arrays higher than 50 MHz, alternative technology, such as MEMS and laser dicing, may need to be exploited. • Other challenges are the high electrical impedance of the small array elements that causes electrical impedance mismatch and the lack of highspeed electronics needed for the development of the HF beam former. HIGH-FREQUENCY IMAGING

23. High-frequency ultrasound has many clinical applications. • In ophthalmology, scanners at 20 MHz or slightly lower have been used to interrogate the posterior components of the eye, including the retina. • Those at 40 MHz and higher are useful for visualizing the anterior segments of the eye. • Clinical applications of HF ultrasound in dermatology include characterization of tumors and assessing the size of structures in the skin. • In ophthalmology and dermatology, HF ultrasound competes with OCT. • The resolution of HF ultrasound is inferior but the depth of penetration is superior. HIGH-FREQUENCY IMAGING

27. Noninvasive imaging of small animals like mice and rats has recently generated a great deal of interest because, for gene therapy, mice or rats are the preferred animal models. • Due to the animals’ small size, clinical imaging devices are not capable of yielding sufficient resolution. • MicroMR, microCT, and microPET designed specifically for small-animal imaging have been developed and are commercially available. • UBM has also been used for small-animal imaging. HIGH-FREQUENCY IMAGING

29. At frequencies higher than 100 MHz, UBM-like devices have been developed for imaging cells and material structures. • These devices are called scanning acoustic microscopes (SAMs). • At a few gigahertz, SAM has a resolution comparable to that of a light microscope and the advantage of being able to penetrate light-opaque media. • It has been used mainly for nondestructive evaluation of materials. • A variation of SAM is the laser scanning acoustic microscope (SLAM), in which the perturbation of the acoustic field generated by an ultrasonic transducer by an object is mapped by scanning a laser. ACOUSTIC MICROSCOPES

30. TANKS FOR ATTENTION