Developing Angle Correction Methods For The Laboratory [continued]

by M. Robert DeJong, RDMS, RDCS, RVT
Society Of Diagnostic Medical Sonographers
Representative to the Board Of Directors

Angle correction must be based on valid grayscale and Color Doppler imaging. Color Doppler may be useful in locating stenotic areas. The spectral Doppler is turned on when ready to obtain a Doppler spectral tracing, freezing the 2D image. The image is frozen after waveforms have been obtained and the angle correction is then fine-tuned as needed. If the transducer was moved to improve the Doppler signal, the 2D image must be updated so that the spectral trace and the 2D image are derived from the same angle. Once the two angles are the same, the image is frozen and angle correction is adjusted.

Angle correcting the Doppler cursor alone, without repositioning the Doppler within the vessel, does not affect the Doppler tracing nor will it improve poor tracings. Setting the cursor to 60 degrees will not cause the sound beam to be at 60 degrees. It only supplies the required mathematical information necessary to allow the machine to calculate the velocity.

Because of the various errors that are inherently introduced when using angles, when a repeat scan is performed it is recommended that prior studies be reviewed. The same angle, or as close to the original angle as possible, should be used. When performing ratios, it is also advisable that the same angles or those within a narrow range be used.

A duplex examination is like a puzzle. It is important to make all the pieces fit together properly and to understand the picture being created in order to accurately answer the clinical question. The relationship between the various modalities used to create the duplex image need to support each other; that is, the grayscale anatomy, the color or power Doppler information, and the spectral Doppler signal all need to lead to the same conclusion. If one of these three parts does not support the diagnosis suggested by the other two, then the reason why needs to be explored. For example, increased velocities of 160 cm/sec are obtained in an internal carotid artery (ICA). This information used by itself indicates a moderate degree of stenosis. However, the grayscale and color/power Doppler images are essentially normal and demonstrate only very minimal plaque. This is a discrepancy that needs to be resolved. Perhaps the increased velocity in the ICA can be explained by noting an elevated velocity of the common carotid artery (CCA). When the ICA/CCA ratio is calculated, it is normal. This finding resolves the isolated increased velocity in the ICA and now the Doppler information correlates with the grayscale and color/power Doppler findings.

Angle correction is a very important aspect of the Duplex examination and needs to be performed carefully and accurately. By reviewing the literature, understanding the above concepts, following the criteria as outlined in the reference being used, and mandating that the entire staff angle correct in the same manner, your laboratory should successfully pass this aspect of the accreditation process and avoid a delayed decision for improper angle correction techniques.

REFERENCES

1. Evans DH, McDicken WN et al: Doppler Ultrasound Physics, Instrumentation and Clinical Applications, John Wiley and Sons, 1989.

2. Madrazo BL, Guy WL, Bendick PJ, Dmuchowski C, Matasar KW, Ranval T. Duplex Diagnostic Criteria: A Survey of ICAVL Accredited Laboratories. Ultrasound in Med Biol; 23 (Suppl 1):S73, 1997.

3. Primovich JF: Should A Constant 60 Degree Angle Or Multiple Angles Be Used In Carotid Duplex Imaging? The Journal of Vascular Technology 17(6):307-310, 1993.

4. Taylor G, Burns P and Wells PNT: Clinical Applications of Doppler Ultrasound, Raven Press, 1988.

5. Rumack CM, Wilson SR, Charboneau JW: Diagnostic Ultrasound, Mosby, 1998.

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