Introduction to Ultrasonography, CT, and MRI

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What is ultrasonography (US), and how does it work?

US is a structural imaging technique that uses high-frequency mechanical ultrasound waves (with frequencies greater than audible sound waves) to create real-time tomographic (cross-sectional) images. A hand-held transducer is applied to a part of the body to transmit ultrasound waves into the patient. Reflected ultrasound waves (echoes) are then detected by the transducer and processed by a computer to create digital images that can be viewed or recorded ( Figure 2-1 ).

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When was US first used for clinical diagnostic purposes?

US was first used as a clinical diagnostic imaging technique in 1942 by Karl Dussik to locate brain tumors and the cerebral ventricles.

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What are some common clinical applications of US?

US is often used for initial evaluation of the abdominal organs (e.g., to evaluate for acute cholecystitis and choledocholithiasis), the pelvic organs (e.g., to evaluate for uterine leiomyomas, ovarian torsion, ectopic pregnancy, endometrial abnormalities, prostate cancer, and testicular torsion), the vessels (e.g., to evaluate for carotid artery stenosis and femoral vein deep venous thrombosis), the thyroid and parathyroid glands (e.g., to assess for thyroid nodules and parathyroid adenomas), and the joints (e.g., to assess for rotator cuff tears) and to serve as a real-time imaging guide during percutaneous biopsies. As US does not involve the use of ionizing radiation, it is also used during pregnancy (e.g., to evaluate the embryo/fetus for congenital anomalies) and in the pediatric setting (e.g., to evaluate the brain and hip joints in infants).

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Why is gel used in US?

Gel is applied between the transducer and skin to displace air and minimize large reflections that would interfere with ultrasound transmission from the transducer into the patient and vice versa. Reflections tend to occur at tissue interfaces where there are large differences in the speed of propagation of ultrasound waves, such as at air/soft tissue and bone/soft tissue interfaces. Gel decreases such reflections at the skin surface by matching the acoustic impedances of the transducer surface and skin surface.

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What is a transducer, and what types are available for US?

A transducer ( Figure 2-2 ) is an electronic device that is used to produce ultrasound waves for transmission into the patient and to receive reflected ultrasound waves (echoes) to create digital images. Lower-frequency ultrasound transducers have a greater depth of tissue penetration, whereas higher-frequency ultrasound transducers have a higher spatial resolution. For general abdominal US, which requires sufficient depth of penetration to image the liver, spleen, and pancreas, a 3- to 5-MHz transducer may be utilized. For US of superficial structures such as the thyroid gland or scrotum, a 10-MHz transducer is often used. Endovaginal transducers are commonly used for gynecologic and early pregnancy examinations, whereas endorectal probes are available for prostate gland examinations.

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What is echogenicity?

Echogenicity is the descriptor of how bright or dark a tissue is on a US image, which depends on whether ultrasound waves are reflected, refracted, attenuated, or transmitted by the tissue. Hyperechoic or echogenic tissues (including air and bone) appear bright, hypoechoic tissues appear dark gray, and anechoic tissues (such as fluid) appear black.

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What is posterior acoustic enhancement?

This is the appearance of increased echogenicity beyond a tissue due to the increased transmission of ultrasound waves, and it is most commonly seen with fluid-filled structures such as cysts or the gallbladder.

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What is posterior acoustic shadowing?

This is the appearance of markedly decreased echogenicity beyond a highly reflecting or absorbing tissue due to the lack of ultrasound wave transmission. This is most commonly due to air, bone, or calcification such as in gallstones. For this reason, US is not used to image air-filled or osseous structures.

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What is Doppler US?

Doppler US allows the imaging and quantification of blood flow velocity in vessels. It is based on the Doppler effect, which refers to the change in sound wave frequency and wavelength that occurs whenever the source of reflected waves (e.g., blood) is moving with respect to the detector (e.g., a transducer). Echoes from blood flowing toward a transducer will have higher frequencies and shorter wavelengths than blood flowing away from a transducer.

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What is computed tomography (CT), and how does it work?

CT is a structural imaging technique that uses x-rays to create tomographic (cross-sectional) images. A patient is placed onto a scanner table and passes through the CT gantry, which contains an x-ray tube and an oppositely located array of x-ray detectors that rotate together about the patient ( Figure 2-3 ). A large number of x-ray projections are obtained from multiple angles at each slice position in the patient, each of which contains data regarding the differential attenuation of x-rays by different tissue types in the patient. These projections are then used by a computer to reconstruct CT images.

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When was CT developed?

CT was developed in the late 1960s and early 1970s by Sir Godfrey Hounsfield in the United Kingdom at EMI Central Research Laboratories, and the first patient brain CT scan was obtained in 1971. Hounsfield, along with Allan Cormack, received the Nobel Prize in Physiology or Medicine in 1979.

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What are some common clinical applications of CT?

CT is commonly used to evaluate patients with neoplastic, infectious, noninfectious inflammatory, and traumatic disorders and as a problem-solving tool to further characterize abnormalities detected on radiography or US. It is especially useful for evaluation of the lungs, airways, bowel, cortical bone, urothelial system, and vasculature.

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