resolution

Question 1

Axial Resolution

Answer 1

Axial resolution is the ability of the ultrasound system to identify two separate objects that lie along the path of the ultrasound beam axis. Axial resolution is determined by the bandwidth of the ultrasound pulse. The bandwidth is the resonant frequencies that are emitted about the center frequency. High-bandwidth pulses are best for axial resolution as they are characterized by high-frequency signals of short duration. As seen in Figure 1.6, short pulses of high-frequency ultrasound offer the greatest axial resolution. A general rule is that the axial resolution of a system is approximately 1.5 times the wavelength of the system. Therefore, for a 7.5-MHz transducer axial resolution is 0.3 mm. Improved axial resolution does not come without a cost. The shorter the pulse, the lower its energy level, so that the penetration and returning echoes are weaker. Similarly, high-frequency sound is quickly attenuated. Accordingly, the echocardiographer must select these parameters based on the imaging needs.

Question 2

Lateral (Azimuth) Resolution

Answer 2

Lateral resolution is the ability of the ultrasound system to distinguish between objects that are horizontally aligned and perpendicular to the path of the ultrasound beam. Beam width is a primary determinant of lateral resolution. Wide beams produce a “smeared” image of two such objects, whereas narrow beams can identify each object individually. Signal frequency and transducer size impact lateral resolution, but for typical cardiac ultrasound transducers the beam width is approximated as depth/50, yielding at 10 cm of depth a beam width of approximately 2 mm.

Question 3

Elevational Resolution

Answer 3

Elevational Resolution
Elevational resolution is the ability of the ultrasound system to distinguish between objects that are vertically aligned and perpendicular to the emitted ultrasound beam. Although 2D images appear to display a thin slice of cardiac anatomy, in actuality the information gathered from the entire thickness of the beam is averaged and displayed. For this reason, the thinner the ultrasound beam, the better the elevational resolution of the system (see Fig. 1.7). Signal frequency and transducer size impact elevational resolution, but a typical cardiac ultrasound transducer has a beam height approximated as depth/30. Accordingly, at 10-cm depth the beam height is approximately 3.3 mm. Note that axial resolution offers fidelity of 50% greater than that achieved in the lateral and elevational planes.

Question 4

Optimizing Resolution

Answer 4

Optimizing Resolution
The interplay of the transducer size, signal frequency, and focal length and the distance of the structure of interest determine beam width and height. The beam is narrowest in the near-field or focal zone and divergent in the far field. Resolution is therefore better in the near field and decreases in the far field. Factors that lengthen the near field, such as a higher transducer frequency and a larger transducer radius, improve lateral and elevational resolution. Focusing further decreases the width of the ultrasound beam and improves lateral and elevational resolution at the focal point. However, focusing often increases beam divergence distal to the focal zone, with an associated loss of lateral and elevational resolution. These factors explain why it is preferable to position a transducer with a relatively high frequency (smaller wavelength) close to the target of interest to optimize both lateral and elevational resolution. More precise measurements are made along the axial plane due to the superior resolution in this orientation.

Question 5

Side Lobes

Answer 5

Unfortunately, in addition to the powerful forwardly directed beam of sound energy produced by linear array transducers, additional beams of sound are emitted that travel off-axis to the main beam (Fig. 1.9; Video 1.1). These extraneous beams of sound, called side lobes, can significantly affect imaging quality because the transducer incorrectly processes their reflections as reflections of the main beam. Consequently, structures off-axis to the imaging plane appear incorrectly located on the 2D image.

Question 6

Grating Lobes

Answer 6

Grating Lobes
Grating lobes are side lobes generated with multielement array transducers. Each crystal of the linear array can be considered a point source of sound emission. When these individual sound waves meet in phase and off-axis to the main beam (constructive interference), a grating lobe is created. The position of a grating lobe is predictable as it is related to the spacing of the crystals and the wavelength of the signal.

Question 7

Side Lobe Artifacts

Answer 7

Both side and grating lobes contain less energy than the main beam and usually do not significantly affect the echocardiographic image. However, when these lobes of energy contact a highly reflective surface (catheter, prosthesis, calcium), sufficient energy can be reflected back to the transducer to create an artifact. The transducer believes these reflections have arisen from the main forwardly directed field and mistakenly displays them together with those from the main beam. To reduce such artifacts, the echocardiographer should minimize gain settings to decrease the likelihood of strong reflections from the weaker lobes. To further differentiate an artifact from a real structure, the field should be imaged from another window. An artifact is not likely to be reproduced in multiple planes.