How to Use a Phantom - Verify Image Quality
One of the most common and most difficult issues you may encounter while working on ultrasound systems is poor image quality. When technologists report a poor image quality problem, they often use subjective terminology such as mushy, grainy and splotchy. These are difficult to troubleshoot. To effectively evaluate an ultrasound system’s imaging capabilities, a phantom is essential. There are several tests that need to be performed when verifying the image quality during a service call. These tests should then be performed regularly on the system during a periodic maintenance to look for degradation over time. Let’s take a look at six tests.
1. Dead Zone (Ring Down Ramp)
This test measures the distance from the face of the transducer to the first echo you can visualize. This dead zone is caused because an ultrasound system cannot transmit and receive simultaneously. The depth of the dead zone is determined by the frequency of the probe (higher frequency = decreased dead zone), performance of the transducer and the transmit/receive portion of the ultrasound system.
2. Geometric Accuracy – Horizontal and Vertical Measurement
Horizontal and vertical measurements verify the accuracy of the caliper measurements. Accurate representation of the depth, volume and size of objects is critical in proper diagnosis. By scanning a phantom using known targets, the accuracy of caliper measurements can be determined. Because vertical distance measurements are based on the timing of the ultrasound image, vertical distance errors are less likely to occur than horizontal measurements.
3. Axial and Lateral Resolution
Resolution is defined as the ability to see two closely spaced objects separately. If a system exhibits poor resolution characteristics, small objects located close to each other will be displayed as one. This can result in misdiagnosis of the ultrasound image. As a general rule, axial resolution is improved with higher frequencies. Lateral resolution is effected by several conditions that include depth, beam width, and focusing capabilities.
4. Focal Zone
The focal zone is the area surrounding a focal point where the intensity of the ultrasound beam and the lateral resolution is the best. From a clinical standpoint, this area is the best location to obtain diagnostic images. The focal zone can be negatively impacted by changes in the transmit/receive circuitry of the ultrasound system or damage to the probe.
5. Maximum Depth of Penetration
Sensitivity is the ability of an ultrasound system to see and display weak echoes from small objects. From a clinical perspective, weak echoes are produced from internal structures of organs. Defining these structures are vital in the evaluation of ultrasound images. Several factors affect the sensitivity of an ultrasound system such as, transmit/receive circuitry, focus of the transducer, attenuation and depth and shape of object. The maximum depth of penetration for an ultrasound system is limited by the output power, TGC, overall gain, transducer frequency and focal zones.
6. Gray Scale
Gray scale processing uses the amplitude of the echoes received and maps them to a gray scale. The ability of the system to display a barely visible (lowest gray scale) echo to the maximum echo is also called the dynamic range of the ultrasound system. Clinicians rely on this dynamic range to visualize various density objects.
In summary, image quality is a difficult problem to define and troubleshoot. By using these tests periodically we are able to evaluate degradation of image quality over time as well as test the quality during a service call.
For more information regarding these tests and how to perform them, register for our Webinar scheduled for December 13 at
www.conquestimaging.com or contact us directly at firstname.lastname@example.org or 866-900-9404.