Spatial resolution determines the degree of image clarity. Resolution is the ability of the ultrasound machine to distinguish two structures (reflectors or scatterers)
that are close together as separate. Spatial resolution is influenced by axial and lateral resolution, both of which are closely related to ultrasound frequency. Axial
resolution refers to the ability to distinguish two structures that lie along the axis (i.e. parallel) of the ultrasound beam as separate and distinct. Axial resolution
is determined by the pulse length. A high frequency wave with a short pulse length will yield better axial resolution than a low frequency wave.
In figure A, a 5 MHz transducer generates ultrasound waves that travel 0.3 mm per cycle (wavelength = 0.3 mm = speed of sound / frequency = 1,540 m/sec divided by 5 x 106 cycles/sec).
The pulse length is the distance traveled by one echo (3 cycles in this case). As seen in figure A, a 5 MHz transducer (wavelength = 0.3 mm and pulse length = 0.9 mm; 3 cycles), the axial
resolution is sufficient to distinguish the 2 target objects as separate because the incident wave hits target # 1 (brown) before hitting target # 2 (green).
In figure B, with a 2.5 MHz transducer (wavelength = 0.6 mm and pulse length = 1.8 mm; 3 cycles), the axial resolution is no longer adequate. Because both target # 1(brown) and
target # 2 (green) are hit by the same wave, both target objects are seen as one.
Lateral resolution refers to resolution of objects lying side by side (i.e., perpendicular to the beam axis). Lateral resolution is directly related to the transducer beam width,
which in turn is inversely related to the ultrasound frequency. A high frequency transducer emits a wave with a short wavelength and a small beam width. Lateral resolution is poor when
the 2 structures lying side by side are located within the same beam width. Because the returning echoes overlap with each other side by side, the 2 structures (1 and 2 in figure) will
appear as one on the display. It is therefore clinically important to choose the highest frequency transducer possible to keep the beam width as narrow as possible in order to provide
the best possible lateral resolution. However, attenuation also increases with frequency thus one must strike a balance between resolution and attenuation.
The beam width can be further reduced by adjusting the focal zone (FZ). Lateral resolution is the best at the FZ, where the beam is narrowest. It is
therefore clinically useful to focus the target structure within the focal zone
to yield the best possible lateral resolution. The beam is known to diverge (increased beam width) as it propagates deep into the far field.