FORMATION OF AN ULTRASOUND IMAGE

Proper visualization and accurate interpretation of an ultrasound image requires a basic understanding of ultrasound components, principles, physics, and terminology. The basic components of an ultrasound machine are listed in Table 1 and include the transmitter to send electrical signals to the transducer, the transducer to interconvert electrical energy and acoustic energy using the piezoelectric effect, the receiver to convert electrical signals into an image, and the monitor to display the image. An optional printer provides a hardcopy image.

Table 1 Components of Ultrasound Machine

There are three essential principles of ultrasonography (Table 2): the piezoelectric effect, pulse-echo principle, and acoustic impedance. Within the transducer, piezoelectric crystals expand and contract to interconvert electrical and mechanical energy, a process known as the piezoelectric effect. When an ultrasound wave contacts tissue, some of the signal is reflected and some is transmitted into tissue. The reflected waves bounce back and contact the crystals within the transducer, generating electrical impulses comparable to the strength of the returning wave. This is known as the pulse–echo principle. Acoustic impedance is the density of tissue multiplied by the speed of sound in tissue. The strength of the returning echo depends on the difference in density between the two structures imaged. Structures of different acoustic impedance (e.g., bile and gallstones) are relatively easy to distinguish from one another, whereas those of similar acoustic impedance (e.g., spleen and kidney) are more difficult to distinguish.¹

Table 2 Essential Principles of Ultrasound

The basic physics of ultrasonography are important for good image formation, and terminology used for ultrasonography is listed in Table 3. Ultrasound waves are high-frequency (>20 kHz) mechanical radiant energy transmitted through a medium. The frequency (number of cycles/second) of medical diagnostic ultrasound is 2.5–10 MHz. As frequency increases, resolution improves, but penetration to deeper tissue decreases. Generally, the highest frequency transducer that produces the best resolution of the target organ is chosen (3.5 MHz for FAST). Common clinical applications of different ultrasound frequencies are listed in Table 4.

Table 3 Ultrasound Terminology

Table 4 Clinical Applications of Selected Transducer Frequencies

Propagation speed (determined by density and stiffness of the medium) is greater in solids than in liquids, and greater in liquids than in gases. Ultrasonic waves travel poorly through gases and therefore the lungs, bowel, and organs underlying areas of subcutaneous emphysema are poorly visualized. Air-filled organs can be visualized when surrounded by liquid, which provides an acoustic window, allowing the passage of the ultrasound waves. Bone appears black on ultrasound because it attenuates sound waves strongly.

The amplitude, or height of a wave, is a measure of its intensity. As ultrasound waves travel through tissue, the amplitude is diminished or attenuated. Lower-frequency waves (3.5 MHz) have greater amplitude and are attenuated less, allowing for greater penetration. Conversely, high-frequency waves (7.5 MHz) are chosen for high resolution of superficial structures, but are unsuitable for deeper structures due to higher attenuation.

Ultrasound waves are attenuated by absorption, scattering, and reflection. Absorption is the conversion of sound waves to heat and scattering is the redirection of waves as they meet an irregular boundary. Reflection is the return of the wave to the transducer. The reflected waves form the image displayed on the monitor. Artifacts in ultrasound imaging, or errors in imaging, are features of the image that do not have precise correspondence to the image being scanned (e.g., shadowing of gallstones).

The degree of amplification or amplitude of returning waves can be adjusted by the gain setting. Increasing the gain will make the displayed image brighter, and conversely, decreasing the gain will make a bright image darker. Ultrasound waves are attenuated as they travel through tissue resulting in fewer and fewer waves penetrating to deep structures. Therefore, fewer ultrasound waves are reflected from deep organs and returned to the transducer. Time-gain compensation (TGC) will increase the amplitude of returning waves from deeper structures, which allows adequate visualization of deeper or thicker organs. TGC allows liver, for example, to appear uniform. Without TGC, the deeper liver parenchyma would appear darker as distance from the transducer increases.

The echogenicity of a structure is defined as the degree to which tissue echoes ultrasonic waves (generally reflected in ultrasound images as the degree of brightness). Tissues that reflect waves strongly will appear bright and are hyperechoic. Tissues that conduct ultrasound waves well are hypoechoic and are darker, while anechoic tissues conduct waves very well and appear black because essentially no waves are reflected back to the transducer. Isoechoic tissue transmits ultrasound similar to that of surrounding tissues, and is displayed with similar intensity (Table 5).¹

Table 5 Terminology Used in Interpretation of Ultrasound Images

TECHNIQUE

The patient’s identifying information is first entered to annotate the hardcopy ultrasound images. With the patient in the supine position, a liberal amount of ultrasound transmission gel is applied to the subxiphoid, left and right upper quadrants, and suprapubic areas. Using four transducer positions as shown in Figure 1, the pericardium and five dependent abdominal regions are examined for free fluid:

Right subdiaphragmatic space

Hepatorenal interface (Morrison’s pouch)

Left subdiaphragmatic space

Splenorenal interface

Pelvis

Figure 1 Transducer positions for focused assessment with sonography for trauma (FAST): 1, pericardial; 2, right upper quadrant; 3, left upper quadrant; and 4, pelvis.

Although Morrison’s pouch was shown by Rozycki and colleagues² to be the most sensitive for free intra-abdominal fluid, all five regions of the abdomen should be examined to maximize sensitivity of the test. Each area should be evaluated in two planes (longitudinal and transverse) with confirmation of positive regions using two views.

The FAST exam begins with the examination of the pericardial area. The gain is adjusted until the blood within the heart appears anechoic (black). Proper gain will ensure that any hemoperitoneum will appear anechoic. Correct superior/inferior and left/right orientation should be checked by noting the position of the visible indicator on the hand-held transducer.

A 3.5-MHz convex transducer is oriented for sagittal sections and positioned in the subxiphoid area directing the transducer superiorly. Often, mild pressure on the transducer below the xiphoid toward the pericardial sac is required to visualize the heart. If this is unsuccessful, a left, parasternal, 4th or 5th intercostal view will be required. Obesity, rib/sternal fracture, subcutaneous emphysema, and a narrow subcostal angle may necessitate the parasternal view, and/or make this part of the examination indeterminate. Hemopericardium is detected by an anechoic band between the heart and the pericardial/diaphragmatic interface, as seen in Figure 2.

Figure 2 Positive pericardial view showing anechoic hemopericardium between liver and heart.

The right upper quadrant is then visualized by placing the transducer in the right mid to posterior axillary line, 11th intercostal space, in both longitudinal and transverse planes to visualize the right subdiaphragmatic and hepatorenal interface. An anechoic band between the liver and kidney as shown in Figure 3 identifies the presence of a minimal amount of blood, and a moderate hemoperitoneum is shown in Figure 4.

Figure 3 Sagittal view of right upper quadrant showing minimal hemoperitoneum between the right kidney and liver.

Figure 4 Sagittal view of right upper quadrant with moderate hemoperitoneum.

The left upper quadrant is examined by directing the transducer between the 10th and 11th ribs in the posterior axillary line. The right sub-diaphragmatic and splenorenal spaces are examined in two planes for free fluid, detected again by an anechoic band separating the two organs. A normal view of the hyperechoic left kidney/spleen interface is shown in Figure 5 and a positive left upper quadrant view is shown in Figure 6.

Figure 5 Normal sagittal view of left upper quadrant showing the hyperechoic left kidney/spleen interface.

Figure 6 Sagittal view of left upper quadrant showing anechoic hemoperitoneum between left kidney and spleen.

Finally, the transducer is placed transversely just above the symphysis pubis and directed inferiorly looking for a coronal view of the bladder. This is ideally done before bladder catheterization to allow for a distended bladder, which optimizes ultrasound transmission and detection of free fluid posterior to the bladder in the rectovesical/uterine space. If a catheter has previously been placed, saline can be injected into the bladder through the catheter, or the catheter can simply be clamped and the pelvic view obtained after passive filling. Free intra-abdominal fluid is best detected on longitudinal plane in the rectovesical or rectouterine space by an anechoic band between the bladder and uterus or rectum as shown in Figure 7.

Figure 7 Positive longitudinal view of pelvis for hemoperitoneum posterior to bladder.

TROUBLESHOOTING

Difficulties in image formation are often solved with the simple techniques and strategy changes found in Table 6. A common solution to improve visualization is to apply a liberal amount of gel and reapply whenever the image quality is poor. An image that is too dark or too bright may require an adjustment in the gain. Poor visualization of deeper structures may also require a lower-frequency (2.5-MHz) transducer or an increase in time-gain compensation of the far field. A 2.5-MHz transducer may be necessary for the obese trauma patient. A 5-MHz transducer may increase the resolution of FAST in pediatric trauma patients, and a 7.5-MHz transducer is optimal for superficial structures (vascular, soft tissue). Subcutaneous emphysema poses a significant problem for ultrasound waves and alternate sites such as the parasternal position may be necessary for adequate visualization.

Table 6 Troubleshooting

One may become disoriented during a FAST exam, and ultrasonographers should remember to reestablish surface anatomy landmarks, confirm transducer orientation (superior/inferior, left/right), and identify obvious internal landmarks (e.g., liver, kidney, spleen). Difficulties visualizing the pericardial sac can be avoided by gentle pressure directing the transducer cephalad under the xiphoid process. Alternatively, moving to the left parasternal view between the 4th or 5th intercostal spaces can be helpful. Transducer movement should be slow with light pressure, watching for the motion of heart contraction. The right or left upper quadrant may be better visualized by moving up or down an intercostal space, moving posteriorly, or during end inspiration. In cooperative patients, requesting an end inspiration breath-hold may be helpful, especially if the shadow of a rib is obscuring a desired view. Oro/nasogastric tube decompression of the stomach may improve visualization of the left upper quadrant. Perhaps the most important maneuver to examine the splenorenal interface and left subdiaphragmatic space is to move the transducer posteriorly, flat on the stretcher, and then point the transducer anterior between the 9th or 10th interspace. The pelvic views should be done with a full bladder.

INDICATIONS

FAST should be performed on all trauma patients who require evaluation of the chest and abdomen and cannot be cleared by physical exam. It should not delay a patient with penetrating abdominal trauma and hypotension or peritonitis from surgical exploration. In the case of penetrating thoracoabdominal trauma, FAST is valuable in early identification of pericardial tamponade or hemoperitoneum. This early application of FAST can direct operative intervention toward the body cavity most likely injured.

FAST is indicated in the evaluation of the unstable, multitrauma patient with an unidentified cause of hypotension. In this scenario, CT is contraindicated and FAST provides a rapid screening test without moving the patient from the resuscitation area. A positive exam is most helpful in this situation and Rozycki et al.³ reported 100% sensitivity and specificity (8 of 8 patients) for intra-abdominal injury in patients with a positive FAST and hypotension. McKenney et al.,⁴ in a prospective evaluation of an ultrasound scoring system, reported similar results. In this series, 10 of 10 patients with initial hypotension (systolic blood pressure <90), and 32 of 36 patients with subsequent hemodynamic deterioration and a significant hemoperitoneum on FAST had a therapeutic laparotomy. Farahmand et al.,⁵ in a study of 128 hypotensive patients suffering blunt abdominal trauma found FAST to be indispensable. The sensitivity of FAST for all injuries was 85%, for surgical injuries 97%, and 100% for fatal injuries. The authors found that FAST was able to virtually exclude surgical injury and detect surgical injury in 64% of positive studies. These studies strongly suggest that the combination of hemoperitoneum and hypotension mandate urgent laparotomy. A negative exam in the multi-injured, hypotensive patient should prompt further aggressive diagnostic and therapeutic evaluation.

The portable and noninvasive nature of ultrasound permits repeat FAST evaluations. A secondary ultrasound is most important in patients without obvious blood loss, a negative primary FAST, and continued hemodynamic instability despite ongoing resuscitation. Secondary FAST may also be performed on patients where CT is unavailable or delayed. Recently, Blackbourne and colleagues⁶