Ultrasonography

Ultrasound is to the Surgeon as the Stethoscope is to the Internist!

Various technologies assist us in our ability to diagnose and create new therapeutic interventions. Today we'll talk about diagnostic ultrasound.

What are advantages of ultrasound compared to other imaging modalities?

Portable
Inexpensive
Quick
Dynamic - Spatially and Temporally
Easily Repeatable
Minimizes Risk to Patient

What are the disadvantages of ultrasound?

Learning curve
Operator dependence
Inability to assess through bowel gas or in obese patients
Inability to assess through bone or calcifications

What is the piezoelectric effect?

The process of converting electric energy into mechanical/acoustical energy, and vice versa. An ultrasonic transducer contains PZT (lead zirconate titanate) crystals that have piezoelectric properties allowing for signal conversion of a mechanical stress into a voltage.

What is the pulse-echo principle?

When a transmitted ultrasound wave (pulse) approaches tissue, part of the signal is reflected and part of the signal is transmitted into tissue. Waves that are reflected back (returning echoes) generate a signal that is interpreted by the transducer.

What is amplitude?

The height of a wave. This decreases (attenuates) as waves travel through tissue or are scattered.

What is frequency?

The amount of wave cycles per second. This is inversely proportional to wavelength.

Diagnostic ultrasound commonly uses frequencies of 2-20MHz.
The upper limit of detection by the human ear is 20kHz.

What is the "Frequency Trade-Off"?

Using high frequency transducers, there is greater spatial resolution.
Using low frequency transducers, there is less signal attenuation.

Because of this, abdominal transducers are generally low frequency (i.e. 3.5MHz curvilinear transducer), with greater depth visualization. A better transducer to visualize a superficial structure like a thyroid nodule might be a 10MHz linear probe.

What is the difference between linear-array and curvilinear-array transducers?

Linear transducers provide more near-field (superficial) information.
Curvilinear transducers have a sector-like field of view that becomes wider with increasing depth.

What is Acoustic Impedance?

A property of the tissue of interest related to its density and the speed of sound through it. Tissues with different acoustic impedance will be distinguishable using ultrasound (i.e. gallstone versus gallbladder). Sound waves reflect at the interface of different acoustic impedances.

Why do you use a coupling gel on the transducer probe at the interface with the patient's skin?

It removes air from the interface and matches the acoustic impedance of the crystal with the contact surface.

What is A-mode ultrasound?

Amplitude Modulation
This is the most basic, one-dimensional form of ultrasound, which records the amplitude of the echo in respect to time. It is now obsolete.

What is B-mode ultrasound?

Brightness Modulation
This is the most commonly used form of ultrasound, converting amplitude into pixel brightness/intensity. The more wave reflection, the greater echo amplitude, the greater the brightness or signal echogenicity.

What is echogenicity?

Characterizes the appearance of tissues based on relative brightness (echo amplitude).
Isoechoic = similar brightness
Hyperechoic = brighter/whiter
Hypoechoic = darker
Anechoic = black

What is M-mode ultrasound?

Movement Modulation
This allows assessment of a moving structure like heart valves, and was used prior to the development of 2D-real time B-mode displays.

How does 2D-real time B-mode ultrasound work?

The transducer contains an array of piezoelectric crystals, or contains a moving crystal. Sequential pulses of ultrasound beams are swept in a plane to create a field of echoes that is reconstructed as a 2D-image.

What is Tissue Harmonic Imaging?

An enhanced form of 2D-real time imaging using a complex form of signal processing that rejects artifacts from scatter.

What is Spatial Compound Imaging?

Another enhanced form of 2D-real time imaging that reduces speckle and graininess by averaging multiple signals.

What is the Doppler Effect?

A change in the received frequency of an observer due to a difference in relative motion between the source and observer. As a consequence, if the source is moving toward an observer it is received with a shift to a higher frequency. If the source is moving away it is received with a shift to a lower frequency. This is helpful to assess blood flow velocity.

What is a Duplex ultrasound?

Combines 2D real-time B-mode ultrasound imaging with color Doppler ultrasound. This allows you to simultaneously visualize the structure of blood vessels or heart valves and see a color correlate of flow velocity through them.

Does red flow mean it is an artery?

No! The red and blue colors with Duplex imaging are arbitrary and refer to positive or negative directionality of flow with reference to the transducer.

What are the basic components of an ultrasound machine?

Monitor
Keyboard
Signal-Processing Unit
Transducer
Image Recorder

What is Time Gain Compensation?

A form of signal processing that compensates for attenuation due to increasing level of depth.

What is the FAST exam?

Focused Abdominal Sonogram for Trauma
It is a rapid survey for assessment for truncal injuries as part of the secondary survey of trauma patients. It can be used to detect hemoperitoneum as an alternative to performance of diagnostic peritoneal lavage in hemodynamically unstable patients. It also surveys for pericardial blood. This may be repeated as necessary.

What are components of the FAST exam?

Using a 3.5MHz curvilinear-array transducer, and ensuring the patient has a full bladder, the following views can be imaged, assessing the heart and the three most dependent regions in the body:
1. Subxiphoid View (r/o pericardial effusion)
2. RUQ View (r/o blood in Morrison's Pouch, hepatorenal recess)
3. LUQ View (r/o blood in the splenorenal recess)
4. Pelvic View

Why do you image the heart first?

It allows a standard to set the gain for detection of blood.

What are common forms of ultrasound use in acute care and ICU settings?

-Localization of vessels for central venous access
-Screening for deep venous thrombosis
-Detection of pleural effusions and performance of thoracentesis
-Diagnosis of acute sinusitis
-Detection of foreign bodies in soft tissue
-Evaluation of patients with abdominal pain (i.e. cholelithiasis)
-Identification of abdominal aortic aneurysms
-Confirmation of reduction of incarcerated hernias
-Evaluation for fascial defect from suspected wound dehiscence
-Detection of abscesses
-Echocardiography

What does Echocardiography assess?

Cardiac Anatomy
Heart wall function
Heart valvular function
Ejection Fraction

What advantages does Transesophageal Echo (TEE) have over Transthoracic Echo (TTE)?

TEE has the transducer in the esophagus, which is closer to the heart, and avoids artifact from the ribs and lungs.
It has improved imaging of the valves and aorta.

What are the US Preventive Services Task Force Recommendations for Screening for Abdominal Aortic Aneurysms?

-Grade B: One-Time screening for AAA using ultrasound in men 65-75y who have smoked.
-Grade C: No recommendation for men 65-75y who have never smoked.
-Grade D: Against routine screening for AAA in women.

What are indications for breast ultrasound?

-Assessment of palpable or vaguely palpable breast mass
-Assessment of lesion detected on mammogram
-Evaluation of nipple discharge with assessment of ducts
-Assessment of dense breast
-Assessment of pregnant or lactating patients
-Follow-up of seromas, hematomas, prostheses
-Assessment of abscesses
-Guide intervention - FNA, Core-needle biopsy, or excisional biopsy

What probe is commonly used for breast ultrasound?

7.5MHz linear-array transducer

What are lesion characteristics to assess with breast ultrasound?

1. Margins
2. Retrotumoral acoustic phenomenon: shadowing, enhancement
3. Echogenicity
4. Compression effect on shape and internal echoes

What does a simple cyst look like on ultrasound?

Sharp, smooth-margined round mass
Homogeneous, anechoic interior
Some compression
Posterior enhancement

What are features of malignant breast lesions on ultrasound?

Indistinct, jagged margins
Few internal echoes
Posterior shadowing
Taller than wide

What are common uses of intraoperative ultrasound (IOUS)?

Hepatobiliary and pancreatic procedures
-detection of stones
-detection of mass lesions
-detection of vessels and ducts
-diagnosing splenic or portal vein thrombosis
-guide biopsy and ablative procedures

IOUS can be done with conventional or laparoscopic transducers.

What type of transducers are commonly used for endoscopic ultrasound?

7-20MHz radial or sector-scanning transducers

Radial transducers provide for a 360-degree visual field
Sector-scanning systems can be easier to perform Doppler and biopsy capabilities.

What type of transducer is used for endorectal ultrasound?

A 7-10MHz radial transducer covered by a water-filled latex balloon.

For TNM staging of a rectal cancer by endorectal ultrasound, what prefix is used?

uTNM = as assessed by EUS
cTNM = as assessed clinically
pTNM = as assessed by pathologist
yTNM = if neoadjuvant chemotherapy was used
mTNM = if multiple primaries were found at a single site
rTNM = for a tumor recurrence
aTNM = if detected at autopsy

Late... SG

Sources:
O'Leary's Physiologic Basis of Surgery, 3rd ed.
Hangiandreou NJ. AAPM/RSNA Physics Tutorial for Residents: Topics in US. B-Mode US: Basic Concepts and New Technology. RadioGraphics 2003; 23: 1019-1033.
Rozycki GS. Surgeon-Performed Ultrasound: Its Use in Clinical Practice. Ann Surg 1998; 228: 16-28.
US Preventive Services Task Force AAA Screening Recommendations (2005)