Ultrasound Studies



Ultrasound Studies






OVERVIEW OF ULTRASOUND STUDIES

Ultrasound (ultrasonography) is a noninvasive procedure for visualizing soft tissue structures of the body by recording the reflection of inaudible sound waves directed into the tissues. The image produced by the ultrasound is called a sonogram. The diagnostic procedure, which requires very little patient preparation, is used in many branches of medicine for accurate diagnosis of certain pathologic conditions (Chart 13.1). It may be used diagnostically with the obstetric, gynecologic, or cardiac
patients and in patients with abnormal conditions of the kidney, pancreas, gallbladder (GB), lymph nodes, liver, spleen, abdominal aorta, bile ducts, ureters, bladder, thyroid, or peripheral blood vessels. Frequently, it is used in conjunction with radiology or nuclear medicine scans. The procedure is relatively quick (often requiring only a few minutes to an hour) and causes little discomfort. No harmful effects have yet been established at the low intensities that are used (<100 mW/cm2). However, as with any diagnostic procedure, ultrasound should not be used frivolously.



Principles and Techniques

Ultrasound uses high-frequency sound waves to produce an “echo map” that characterizes the position, size, form, and nature of soft tissue organs. Echoes of varying strength are produced by different types of tissues and are displayed as a visual pattern after computer processing of the echo information. The capability of acquiring real-time images means that ultrasound can readily demonstrate motion, as in the fetus or the heart. Ultrasound, however, cannot appropriately image air-filled structures such as the lungs.


Doppler Method

A phenomenon that accompanies movement, the Doppler effect, can be combined with diagnostic ultrasound imaging to produce duplex scans. Duplex scans provide anatomic visualization of blood vessels and a graphic representation of blood flow characteristics. Flow direction, velocity, and the presence of flow disturbances can readily be assessed. Certain equipment is capable of advanced Doppler imaging techniques, such as:



  • Color Doppler imaging provides a color-coded depiction of selected blood flow parameters.


  • Doppler energy, power Doppler, or color angio is sensitive to very low blood velocity states and is often used to evaluate blood flow through solid organs.


  • B-flow Doppler images the blood itself, producing images that resemble an angiogram.

These techniques establish the patency of a given blood vessel and are useful in investigating perfusion to an organ or mass. They are also helpful in evaluating complications in transplanted organs.



Implications of Ultrasound Studies


Benefits and Risks



  • Ultrasound is a noninvasive procedure with no radiation risk to patient or examiner.


  • It requires little, if any, patient preparation and aftercare.


  • The examination can be repeated as often as necessary without being injurious to the patient. No harmful cumulative effect has been seen.


  • Ultrasound is useful in the detection and examination of moving parts, such as the heart.


  • It does not require the injection of contrast materials or isotopes or ingestion of opaque materials.


Disadvantages



  • An extremely skilled examiner is required to operate the transducer. The scans should be read immediately and interpreted for adequacy. If the scans are not satisfactory, the examination must be repeated.


  • Air-filled structures (e.g., the lungs) cannot be studied by ultrasonography.


  • Certain patients (e.g., restless children, extremely obese patients) cannot be studied adequately unless they are specially prepared.


Difficult-to-Study Patients

The following general categories of patients may provide some difficulties in ultrasound studies:



  • Postoperative patients and those with abdominal scars: The area surrounding an incision is to be avoided whenever possible. If a scan must be performed over an incision, the dressing must be removed and a sterile coupling agent and probe must be used.


  • Children and agitated adults: Because the procedure requires the patient to remain still, some patients may need to be sedated so that their movements do not cause artifacts.


  • Obese patients: Certain patients cannot be studied adequately in any case. For example, it may be difficult to obtain an accurate scan on a very obese patient, owing to alteration of the sound beam by fatty tissue.


Interfering Factors



  • Barium has an adverse effect on the quality of abdominal studies, so sonograms should be scheduled before barium studies are done.


  • If the patient has a large amount of gas in the bowel, the examination may be rescheduled because air (bowel gas) is a very strong reflector of sound and does not permit accurate visualization.


OBSTETRIC AND GYNECOLOGIC ULTRASOUNDS


• Obstetric Sonogram

Ultrasound studies of the obstetric patient are valuable in (1) confirming pregnancy; (2) facilitating amniocentesis by locating a suitable pool of amniotic fluid; (3) determining fetal age; (4) confirming multiple pregnancy; (5) ascertaining whether fetal growth is normal, through sequential studies;
(6) determining fetal viability; (7) localizing placenta; (8) confirming masses associated with pregnancy; (9) identifying postmature pregnancy (increased amount of amniotic fluid and degree of placental calcification); (10) serving as a guidance method for chorionic villus sampling (CVS), embryo transfer, intrauterine device (IUD) extraction, and percutaneous umbilical vein sampling (PUVS); and (11) determining fetal nuchal translucency (FNT). A pregnancy can be dated with considerable accuracy if an ultrasound is done at 20 weeks’ gestation and a follow-up scan is done at 32 weeks’ gestation. This validation is most important when early delivery is anticipated and prematurity is to be avoided. Conditions in which determination of pregnancy duration is useful include maternal diabetes, Rh immunization, and preterm labor (Chart 13.2).

The pregnant uterus is ideal for echographic evaluation because the amniotic fluid-filled uterus provides strong transmitting interfaces between the fluid, placenta, and fetus. Ultrasonography has become the method of choice for evaluating the fetus and placenta, eliminating the need for the potentially injurious radiographic studies that were used previously.


Reference Values


Normal Obstetric Sonogram

Normal image of placental position, size, and structure

Normal fetal position and size with evidence of fetal movement, cardiac activity, and breathing activity

Adequate amniotic fluid volume

Normal fetal intracranial, thoracic, and abdominopelvic anatomy; four limbs visualized





Interfering Factors



  • Artifacts may be produced when the transducer is moved out of contact with the skin. This can be resolved by adding more coupling agent to the skin and repeating the scan.


  • Artifacts (reverberation) may be produced by echoes emanating from the same surface several times. This can be avoided by careful positioning of the transducer.


  • A posterior placental site may be difficult to identify because of the angulation of the reflecting surface or insufficient penetration of the sound beam owing to the patient’s size.



• Fetal Echocardiography (Fetal Doppler)

Fetal echocardiography is performed after the detection of a potential cardiac abnormality during an obstetric sonogram or in patients with a strong history of congenital cardiovascular disease. Additionally, women exposed to cardiac teratogens are usually advised to have this study. Not a screening procedure, fetal echocardiograms are most commonly performed in specialized laboratories or teaching hospitals. The heart is imaged in numerous planes, using pulsed Doppler and M-mode tracings, similar to an electrocardiogram (see Cardiac Ultrasound, page 892). Valves and other cardiac structures are measured, and blood velocities and volumes are calculated. Optimal fetal echocardiographic studies are performed between 18 and 22 gestational weeks. Before 18 weeks, the fetal heart is too small, and after 22 weeks, image quality may be degraded by overlying structures.


Reference Values


Normal Sonogram

Normal structure of heart and great vessels

Normal heart rate and rhythm, with proper hemodynamic flow through heart and great valves




Interfering Factors



  • Artifacts may be produced when the transducer is moved out of contact with the skin. This can be resolved by adding more coupling agent to the skin and repeating the scan.


  • Artifacts (reverberations) may be produced by echoes emanating from the same surface several times. This can be avoided by careful positioning of the transducer.


  • A posterior placental site may be difficult to identify because of the angulation of the reflecting surface or insufficient penetration of the sound beam owing to the patient’s size.



• Pelvic Gynecologic Ultrasound; Pelvic (Uterine Mass) Ultrasound Diagnosis; Intrauterine Device (IUD) Localization

The pelvic gynecologic ultrasound study examines the area from the umbilicus to the pubic bone in women. It may be used in the evaluation of pelvic masses to determine the position of an IUD, to evaluate postmenopausal bleeding, or to aid in the diagnosis of cysts, tumors, abscess, fibroids,
cancer, or thickened endometrium. Information can be provided on the size, location, and structure of masses. Spectral or color Doppler can be applied to pelvic vessels, demonstrating normal flow changes associated with the menstrual cycle, and can evaluate abnormal flow patterns to masses/tumors. The examination cannot provide a definitive diagnosis of pathology but can be used as an adjunct procedure when the diagnosis is not readily apparent. It is also used in treatment planning and follow-up radiation therapy for gynecologic cancer. Additionally, follicle development after infertility treatment can be monitored.

This test may be performed by a transvaginal method whereby a slim, covered, lubricated transducer is gently introduced into the vagina. A full bladder is not required. Because the sound waves do not need to transverse abdominal tissue, exquisite image detail is produced. This approach is most advantageous for examining the obese patient, the patient with a retroverted uterus, or the patient who has difficulty maintaining bladder distention. The transvaginal method is the approach of choice in monitoring follicular size during fertility workups and during aspiration of follicles for in vitro fertilization.

For pelvic sonograms using the transabdominal approach, a full bladder is necessary. The distended bladder serves four purposes: It acts as a “window” for transmission of the ultrasound beam; it pushes the uterus away from the pubic symphysis, thereby providing a less obstructed view; and it may be used as a reference for comparison in evaluating the internal characteristics of a mass under study.


Reference Values


Normal Sonogram

Normal pattern image of bladder, uterus, fallopian tubes, vagina, and surrounding structures

Normal Doppler flow patterns of major pelvic blood vessels





Interfering Factors



  • Severe obesity, intestinal gas, or barium in the intestine from recent procedures.


  • The success of a transabdominal scan depends on full bladder distention.


Sep 25, 2018 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Ultrasound Studies
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