FIGURE 131.1 On the left is a low-frequency, phased array probe commonly used in cardiac or abdominal applications On the right is a high-frequency, linear probe used for procedures and identifying superficial structures ULTRASOUND BASICS Ultrasound Physics A complete review of ultrasound physics is beyond the scope of this chapter Knowledge of the basic principles, however, will lead not only to a higher comfort level with machine operation, but also to a greater facility with image acquisition and interpretation Ultrasound refers to sound waves with frequency greater than 20,000 Hz (0.02 MHz, the upper range of audible sound) Frequencies used in diagnostic and procedural ultrasound generally range from to 15 MHz The general principle of diagnostic ultrasound is the pulse-echo effect Sound waves are generated from the transducer and sent into a medium (the body) The transducer then “listens” for the return, or echo, of that sound The emitted sound wave encounters different tissues, with different densities and different distances from the surface of the skin Some of the sound is reflected back to the transducer footprint Once the transducer “hears” a returning echo, the ultrasound system calculates the distance of an object from the transducer based on the amount of time it took the echo to return The intensity of the returning sound wave determines the grayscale assignment on the image For example, fluid does not reflect sound waves at all, thus a fluid-filled bladder will appear black or anechoic (i.e., the transducer did not “hear” any sound waves reflected back) The diaphragm, however, is highly reflective and appears bright on the screen (Fig 131.2 ) Each pixel on the screen is generated in this fashion, thus creating an overall image FIGURE 131.2 A: A full bladder Note that fluid is anechoic (black) B: Morison pouch Note the bright reflection of the diaphragm adjacent to the liver (arrow ) Basic Controls Ultrasound machines, upon first glance, can appear daunting There are many buttons and knobs, and the inexperienced user can often be disheartened Keeping in mind the basic sonography concepts, however, and understanding that these controls simply allow one to adjust and optimize the image, should prevent even the most novice sonographer from feeling intimidated Gain refers to the intensity of the returning echoes on the display screen Adjusting the gain essentially changes the brightness without improving the quality of the image Depth of the image can be adjusted as well For superficial structures, decreasing the depth allows for greater image quality, larger images of the structure under scrutiny, and less wasted space on the screen Increasing the depth allows for visualization of deeper structures (i.e., farther away from the skin and transducer) The freeze button allows the sonographer to hold a still image, usually for purposes of studying an image, performing measurements, or saving On most machines, several seconds of memory are saved and can be toggled forward and backward to find a desired image seen moments before Other control panel functions on ultrasound machines vary widely, but often include color, Doppler, motion-mode (M-mode), focus and tissue harmonics As the practitioner gains more experience, these machine capabilities will become more familiar and allow for more advanced applications General Scanning Techniques One of the greatest advantages to ultrasound is that it is dynamic, with the ability to capture images in multiple imaging planes from multiple orientations This also can lead to confusion when reviewing and interpreting images For that reason, standard and consistent orientation should be used when performing scans Generally speaking, the standard ultrasound examination is performed with the sonographer on the right side of the patient’s bed The probe should be held in the right hand and ultrasound system adjustments made with the left hand during scanning All transducers have some marking that correlates with a dot (or some other identifier) on the monitor screen (Fig 131.3 ) By convention, the transducer marker is always kept to the patient’s right in transverse views or head in sagittal and coronal views and the dot on the screen is located at the top, left side of the monitor (with the exception of cardiac scanning) Adhering to this principle allows for uniformity of images and makes interpretation and review more seamless Objects that are closer to the probe marker appear closer to the dot on the monitor, and vice versa