9.4 Ultrasound Imaging Fundamentals

Ultrasound Imaging Fundamentals

Diagnostic ultrasound, or ultrasonography, is a non-invasive imaging modality that utilizes high-frequency sound waves to produce real-time, dynamic images of soft tissues and internal organs. Unlike radiography, ultrasound does not use ionizing radiation, making it a safe diagnostic tool for both patients and veterinary personnel.

Principles of Ultrasound Mechanics

Ultrasound imaging operates on the principle of echo reflection. The ultrasound machine sends high-frequency sound waves (typically 2.5 to 15 megahertz [MHz]) into the patient's body. As these sound waves travel through tissues, they encounter boundaries between tissues of different densities and acoustic properties. Some sound waves are reflected back to the probe as echoes, while others continue traveling deeper into the body.

Wave Interactions and Artifacts

As sound waves travel through tissue, they experience several interactions:

• Attenuation: The gradual loss of intensity as sound waves travel deeper, due to absorption (energy converted to heat) and scattering. This explains why lower-frequency probes are required to image deep tissues.
• Acoustic Shadowing: Highly reflective or absorptive structures (like bone or calculi) block the passage of sound waves. This creates a clean black void (a shadow) distal to the structure, which is a key diagnostic indicator for bladder stones.
• Acoustic Enhancement: Fluid-filled structures (like the gallbladder or urinary bladder) cause very little attenuation of the sound beam. Consequently, the tissues directly behind these fluid structures receive a stronger sound beam and appear brighter (hyperechoic) on the screen.

Echogenicity (Image Brightness)

The strength of the returning echoes determines the brightness of the image on the screen, described using the following terms:

• Anechoic (Black): Tissues that reflect no sound waves. Sound passes through them completely. Fluid-filled structures like urine in the bladder, bile in the gallbladder, or free abdominal fluid appear completely black.
• Hypoechoic (Dark Grey): Tissues that reflect few sound waves, appearing darker grey than surrounding tissues. Examples include normal lymph nodes, the renal medulla, and some cellular soft-tissue masses.
• Hyperechoic (Bright White): Tissues that reflect a high proportion of sound waves, appearing bright white. Examples include bone surfaces, bladder stones (calculi), organ capsules, and air-tissue interfaces.
• Isoechoic: Tissues that have the same brightness and texture as surrounding tissues.

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Transducers and Probes

The transducer (probe) is the handheld device that sends and receives sound waves. It contains piezoelectric crystals, which expand and contract when electrical voltage is applied, generating sound waves. Conversely, when returning echoes strike the crystals, they generate electrical signals that the machine translates into an image.

Frequency vs. Penetration

• High-Frequency Probes (7.5 - 15 MHz): Provide excellent image resolution (detail) but have poor depth penetration. They are used for small patients (cats, small dogs) or superficial structures (eyes, thyroid, tendons).
• Low-Frequency Probes (2.5 - 5 MHz): Provide deep tissue penetration but lower image resolution. They are used for large dogs or deep abdominal scans.

Common Probe Designs

• Linear Array: Produces a rectangular image, ideal for superficial tissues.
• Convex (Curved) Array: Produces a sector-shaped image with a wide field of view, ideal for general abdominal scans.
• Phased Array (Sector): Has a very small footprint and sweeps the sound beam in a fan shape, making it ideal for scanning between the ribs (echocardiograms/cardiac scans).

Handling and Care of Transducers

Transducers are highly delicate and represent the most expensive component of the ultrasound system.

• Protect Against Drops: A drop can shatter the fragile piezoelectric crystals inside, resulting in permanent dead zones (visible as vertical black lines across the display image).
• Avoid Harsh Chemicals: Probes must never be autoclaved or exposed to high heat. The rubber acoustic membrane on the face of the probe is highly sensitive and can be degraded by alcohol, bleach, or iodine. Probes should be wiped clean of gel immediately after use and disinfected using approved wipes or mild soap and water.

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Patient Preparation

Proper patient preparation is critical for diagnostic ultrasound because air reflects 100% of ultrasound waves, preventing them from entering the body.

1. Clipping (Shaving): Hair traps air, which blocks the sound waves and creates a dark shadow on the screen. A close clip using a #40 clipper blade is required over the entire scanning area. For abdominal scans, this is typically from the xiphoid process to the pubis, extending laterally to the flanks.
2. Skin Cleaning: Apply isopropyl alcohol to the clipped skin to remove oils, dander, and loose hairs. This enhances skin-to-probe contact.
3. Acoustic Gel: Apply a generous layer of ultrasound transmission gel. The gel acts as a coupling agent to eliminate air pockets between the skin and the transducer face.

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Restraint and Assistant Role

The veterinary assistant plays a vital role during an ultrasound examination, handling patient restraint and clinical support:

• Abdominal Restraint: The patient is typically placed in a padded, V-shaped foam trough in dorsal recumbency (on their back). The assistant holds the forelimbs forward and the hind limbs back, keeping the abdomen accessible.
• Echocardiogram Restraint: The patient is held in lateral recumbency on a specialized table with a cutout. This cutout allows the sonographer to place the probe from underneath, scanning directly through the chest wall.
• Panting Management: Dogs often pant when stressed, which causes rapid diaphragmatic movement, pushing abdominal organs in and out of the field of view. The assistant must use low-stress restraint techniques, keep the room quiet and dark, and gently close the patient's muzzle briefly if requested by the clinician to stop panting.
• Clinical Support: The assistant may assist by typing patient demographics into the ultrasound machine console, saving/freezing images on command, preparing slides and syringes for ultrasound-guided fine-needle aspirates (FNA), and cleaning the acoustic gel off the patient's skin after the scan (gel is sticky, and patients will lick it off, which can cause mild self-limiting gastrointestinal upset).

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Radiography vs. Ultrasound: A Comparison