HST.563
LAB 4: Echocardiographic Imaging
Robert Manzke, Ph.D.
Contents
1. Introduction
2. Lab preparation
a. Technical principles of ultrasound
b. Clinical aspects of echocardiography
3. Lab exercise
4. Questions
5. Directions to the lab
6. References
1. Introduction
This lab is an introduction to echocardiographic imaging. Echocardiography is a non-invasive (transthoracic echo,TTE) or minimally-invasive (transesophageal echo,TEE; intracardiac echo, ICE; intravascular ultrasound, IVUS) imaging technique which allows for real-time visualization of the cardiac anatomy and function. Echocardiographic imaging is the clinical standard for functional assessment of the heart and is the firstline modality for assessing heart anatomy, structure, and function. It provides a wealth of essential clinical information such as the size and shape of the heart, its pumping capacity (Ejection Fraction, EF), as well as the location and extent of any damage to its tissues. Due to the real-time character and fine spatial resolution of ultrasound, it is especially useful for assessing diseases of the heart valves. Doppler imaging techniques allow physicians to detect abnormalities in the pattern of blood flow, such as the retrograde flow of blood through leaky heart valves, known as regurgitation. By imaging the motion of the heart wall, echocardiography can help detect the presence and assess the severity of coronary artery disease and myocardial infarcts. Echocardiography is also used to detect pathologies such as hypertrophic cardiomyopathy in which the walls of the heart thicken in an attempt to compensate for heart muscle weakness. A limitation in 2-dimensional ultrasound is that the true three-dimensional anatomy is not captured. Recently, new 3-dimensional ultrasound imaging techniques have been introduced, which give a more complete picture of the anatomy in its entirety.
In preparation for this lab, you will read three review papers on echocardiography [1-3] which summarize recent technological and clinical developments. Focus on [1]. For further reading, we recommend [4] for technical aspects of ultrasound imaging and [5] for the clinical applications, as well as material readily available on the web.
2. Lab preparation
a. Technical principles of ultrasound
Ultrasound waves used in medical applications are primarily longitudinal waves of mechanical pressure, which propagate in a medium such as human tissue (though more recently, medical research into shear waves for ultrasound-based strain imaging has also been extensive). The wave equation for pressure fields in liquids and gases is given by
where pressure is given by , the density of the medium is represented by , the time by t and the bulk modulus by .
The wave propagation velocity is where is the wavelength and is the frequency of the pressure wave. The sound impedance is defined as ,
which represents the ratio of the stress force per unit area to the displacement velocity (Table 1 shows typical values for different biological tissues).
The intensity reflection and transmission factors are given by the expressions
.
The ratio of a reflected signal to the source signal is often expressed in decibels
Table 1: Ultrasound parameters.
Material / Density in kgm-3 / Propagation velocity in m/s / Impedance in10-6 kgm-3s-1
Water / 1000 / 1480 / 1.48
Muscle / 1080 / 1580 / 1.7
Fat / 900 / 1450 / 1.3
Brain / 1050 / 1540 / 1.6
Blood / 1030 / 1570 / 1.58
Bone / 1850 / 3500-4300 / 6.5-8.0
Air / 1.3 / 330 / 4´10-4
Note the very large differences between the soft tissues and bone and air.
Signal reflection factors at boundaries between soft tissues are small:
e.g. fat to muscle .
Reflection factors between soft tissues and bone or air are large:
e.g. bone to brain .
The intensity of the ultrasound wave decreases exponentially with the penetration depth. The attenuation coefficient can be used to determine the total attenuation in dB/cm in the medium according to
At an ultrasound frequency of 1MHz, the attenuation is on the order of 1 dB/cm for soft tissue such as kidney or liver (see Table 2), whereas for bone the coefficient is in the order of 20 dB/cm. The spatial resolution is dependent on the wavelength. A detectable echo occurs if the structural distance is at the order of half the wavelength.
Table 2: Penetration depth and resolution in soft tissue.
Frequency f in MHz / Approximate wavelength in mm / Depth in cm / Typical application3.5MHz / 0.44 / 15 / Liver, Heart
7.5MHz / 0.21 / 7 / Prostate
15MHz / 0.1 / 1 / Intravascular applications
In a practical setup, ultrasound waves are produced and detected by piezoelectric crystals, which change dimensions depending on an applied alternating electrical field. In the simplest setup, a single transducer element transmits a pulse into the tissue (see Figure 1). Immediately after transmission, the transducer is switched to receive mode and reflected waves are detected, forming an image. This simplistic setup is used for 1-dimensional A-mode imaging.
Figure 1: Simple A-line one element setup.
If a time series of A-mode images is recorded, an M-mode image is obtained (see Figure 2).
Figure 2: M-mode image of the heart. Upper row shows A-lines.
If the single element transducer is physically swept along the one spatial axis, one can form a 2-dimensional image (B-mode, see Figure 3).
Figure 3: Mechanical movement of the single element transducer (B-mode).
In most cardiac applications today, electronically-steered phased arrays are used to steer the focus of the ultrasound transducer to sample a 2D image slice through the volume.
Such phased arrays typically consist of several piezo-electric elements organized into a one-dimensional array. Each element of the phase array can be pulsed with a certain transmission delay. This way, the focus of the resulting wave field of all elements can be swept around a scan sector as shown in Figure 4.
Note that the smaller the imaging sector is, the faster echoes can be received and, hence, the higher the frame rate and temporal resolution is. Likewise, the shorter the imaging depth, the higher the pulse repetition frequency, the higher the frame rate and associated temporal resolution.
If the phased array is extended into a two-dimensional plane of elements, the focus can be steered in three dimensions to sweep a conical volume. This is the basis for modern 3-dimensional ultrasound transducers.
Figure 4: Beam forming of phased array and B-mode scanning.
Another important aspect of ultrasound imaging is its Doppler imaging capability. When a wave is reflected or scattered from a moving object, the apparent frequency received by the transducer is changed. This is called the Doppler effect.
A wave scattering object (e.g. blood cell) is moving at vs at angle q.
The frequency of the reflected signal is ,
so the Doppler shift from the transmitted frequency is
Doppler shift frequencies are typically detected in the 0 - 1.3 kHz and 0 - 8 kHz ranges.
Imaging of Doppler frequency versus position can give useful indications of arterial blood flow in tissue regions and major organs. Also valvular function such as mitral valve regurgitation is assessed using Doppler methods.
Figure 5: Color-Doppler imaging of the mitral valve, for assessment of regurgitation.
b. Clinical aspects of echocardiography
Ultrasound imaging finds applications in all clinical fields. Here, we will focus on the subspecialty of echocardiography, imaging of the heart using ultrasound. Depending on the examination type, the echocardiographer performs transthoracic (TTE) acquisition of standard views.
Typical TTE views are (see Figure 6):
(1) Parasternal view
a. Long-axis
b. Short-axis (basal, MV-level, papillary muscle level, apical)
(2) Apical view
a. Four-chamber
b. Two-chamber
c. Long-axis
d. Five-chamber
(3) Subcostal
a. Four-chamber
b. Short-axis (basal, mid-ventricular)
(4) Suprasternal
a. Aortic arch short and long-axis
(5) Right parasternal
a. Ascending aorta
Figure 6: View nomenclature in echocardiography, taken from [5].
More invasive versions of echocardiographic imaging include transesophageal imaging (TEE, see Figure 7), intracardiac echo (ICE, see Figure 8) and intravascular ultrasound (IVUS, see Figure 9). With TEE, a transducer is placed down in the esophagus close to the heart with the patient under anesthesia. TEE and ICE imaging enable acquisition of different views of the heart with significantly fewer artifacts such as rib shadowing, lung artifacts, etc.
Figure 7: TEE probe, taken from [5].
Figure 8: ICE probe, taken from [5].
IVUS imaging uses higher frequencies in the range of 15-40MHz to image vascular walls and atherosclerotic plaques within arteries.
Figure 9: IVUS image of plaque (c) , taken from [5].
Once images are acquired, different quantitative analyses can be performed. Linear measurements of the left ventricle such as internal dimensions, fractional shortening,
wall stress, volume and velocity can be obtained. 2-dimensional measurements can include short-axis area, four-chamber ventricular area, left ventricular volume, stroke volume,
ejection fraction and ventricular mass. Image-processing techniques enable automated measures of cardiac parameters such as EF, strain, and tissue velocity from acquired echo sequences.
2-dimensional imaging, however, limits the accuracy of measurements due to limitations inherent to 2-dimensional slice interrogation within a 3-dimensional object (e.g. off-axis foreshortening). 3-dimensional echocardiography aims at improving this shortcoming by visualizing true 3-dimensional anatomy in a single acquisition.
3. Lab exercise
a. We will introduce different ultrasound probes used for specific medical imaging applications
b. We will evaluate different scan modes such as 2D and 3D imaging, tissue and color Doppler, and play with acquisition parameters such as depth/ gain/ mechanical index etc.
c. We will evaluate a data set with a quantitative evaluation software called Qlab, assessing, for example, regional wall motion and EF
d. We will acquire some example data sets with TTE, aiming at typical views based on volunteer scanning
e. Please take notes to answer the following questions
4. Questions
Part (a) of the questions can be answered using this preparation material. Part (b) can be answered most likely after attending the lab exercise.
(a)
Q1: Why is 2-dimensional ultrasound limiting correct quantitative assessment of the heart geometry / sizes of specific structures, sketch?
Q2: What type of equation is the wave equation?
Q3: What is limiting the frame rate in ultrasound?
Q4: What implications does it have that reflection factors are high at boundaries such as bone brain and small at soft tissue boundaries?
Q5: What ultrasound parameters have to be considered given a certain required penetration depth?
Q6: Which ultrasound parameter is important to choose correctly given a required spatial resolution?
Q7: What is a major limitation in 3-dimensional ultrasound?
Q8: Explain why higher frequencies are used for IVUS imaging.
Q9: What temporal resolution is desirable for a 10 phasic echo loop given a heart rate of 60bpm?
Q10: What is the theoretical spatial resolution of a 7.5MHz ultrasound wave in soft-tissue?
(b)
Q11: What are typical TEE views?
Q12: Explain why views are called two, three, four, five chamber, explain terms such as short axis apical, medial and basal. Sketch a set a typical views and explain terms.
Q12: Which views are easier to obtain (TTE) and why?
Q13: Explain the difference between CW and PW Doppler imaging.
Q14: Explain why TEE and ICE deliver images generally with less artefacts.
Q15: Define the term ejection fraction (EF) and describe a 2-dimensional method to measure it.
Q16: Define and sketch how stroke volume can be measured using 2-dimensional methods.
Q17: How can one obtain information about tissue elasticity using ultrasound?
5. Directions to the lab
The lab will be held April 2nd, 2008 at the main MGH campus. Meeting time 3.45pm at Yawkey Center (MGH) Coffee Central (see map).
6. References
1. Hung, J., et al., 3D echocardiography: a review of the current status and future directions. J Am Soc Echocardiogr, 2007. 20(3): p. 213-33.
2. Salgo, I.S., Three-dimensional echocardiographic technology. Cardiol Clin, 2007. 25(2): p. 231-9.
3. Weyman, A.E., The year in echocardiography. J Am Coll Cardiol, 2007. 49(11): p. 1212-9.
4. Duck, F.A., A.C. Baker, and H.C. Starritt, Ultrasound in Medicine. 1998: Institute of Physics Pub.
5. Feigenbaum, H., Echocardiography. 1994: Lea & Febiger Philadelphia.