Resolution

Obtaining a clear and cystal image is critical in ultrasound, especially when looking for a particular mass in an internal organ. Proper adjustments must be made in order to avoid artifactual echoes from arising so the diagnosis can be accurate and precise. Therefore, the concept of Image Resolution must be appreciated and understood. There are three types of Resolution: Detail, Contrast, and Temporal. Detail Resolution, or Spatial Resolution, can be broken down into Axial and Lateral Resolution. Contrast Resolution refers to Image Brightness (i.e. Output Power, Gain), whereas Temporal Resolution pertains to Time and and the Frame Rate.

Axial Resolution, or LARRD, is the ability of the system to distinguish two structures located at the same direction of the sound beam. LARRD is proportional to half of the Spatial Pulse Length (SPL), and as SPL decreases then LARRD decreases, which is essential to improve image resolution. The dominant factor in determining LARRD is SPL. Another way to reduce SPL is to increase the damping or backing material (make it thicker) or the frequency of the transducer. As more damping or backing material is applied to the thin crystal element, this reduces the number of cycles in a pulse thereby generating a shorter wavelength or SPL. The same applies by changing from a low frequency transducer to high frequency, which can also reduce the wavelength or SPL thus generating a shorter pulse. As previously mentioned, shorter pulses improve image resolution, therefore, in order to decrease LARRD, backing/damping material and the frequency should be increased to improve image resolution.

Lateral resolution, or LATA, is the ability of the system to distinguish two structures located from side to side or perpendicular to the beam. LATA is equal to the beamwidth in the scan plane. As with LARRD, LATA is improved as it decreases, however, the image can only be improved through "focusing". Focusing can be done by applying lens outside of the cystal within the transducer (external), using a curved cystal element (internal), or electronic focusing through Phasing or Time-Delay. Electronic focus is the ability of the transducer to focus the beam by "steering" the sound through phasing. In addition to time delay, greater curvature in the phase delay pattern will have greater time delays thus moving the focus closer to the transducer. On the other hand, less curvature will generate shorter time delays thus moving the focus deeper. Therefore, phasing and time delay provides electronic control of the location of the focus.

Focusing improves LATA, but in Contrast Resolution, the image is improved by a change in the following parameters: output power, gain, time gain compensation (TGC), post-processing, and log compression (log Comp). Contrast resolution refers to image brightness, and if the image is too bright, then one or more of the following parameters are used to change the contrast of the image. For example, by changing the log comp (dB), the number of bits are increased thereby producing a dark image. Contrast resolution are broken down into two types: high contrast and low contrast. In high contrast, a bright image is produced, speckle, increased frame rate, low persistance, and narrow dynamic range. As a result, a bad resolution is obtained. High contrast are often generated for echo studies, however, Low contrast is of the opposite. In Low contrast, a dark image is produced, less speckles, slower frame rate, high persistance, and wide dynamic range. As a result, good resolution is obtained. An example of using Low Contrast resolution is during an abdominal scan.

During Low Contrast Resolution, frame rate is reduced thus degrading Temporal Resolution. Temporal Resolution is the ability of a display to distinguish closely spaced events in time and to present rapidly moving structres correctly. Therefore, Temporal Resolution improves as Frame Rate (FR) increases, and this can be seen by High Contrast Resolution. Temporal Resolution is expressed in milliseconds, and describes the time required to generate one complete frame. Each frame is made of many scan lines, and as the Lines Per Frame (LPF) increases, then FR decreases thus degrading Temporal Resolution. On the other hand, as LPF decreases, then FR increases thus obtaining improved Temporal Resolution.

FR is equal to PRF but inversely proportional to "n" multiplied by LPF. As more scan lines are present, FR is decreased due to the increased time for the reflected echo to come back to the sound source. However, another factor can affect FR, and this is the size of the sector beam. That is, as sector size decrease, then the scan lines on each frame are reduced thereby increasing FR. As a result, Temporal Resolution is improved. Also, by adding more focal points (multifocus), FR decreases thus Temporal Resolution gets degraded, but by reducing focal points, Temporal Resolution increases but LATA gets degraded.