DK2985_C000 1..28 - AlSharqia Echo Club

138 Transesophageal Echocardiographysystem for mapping blood flowing towards the transducer.The velocity scale is surmounted by a positive number correspondingto the maximal velocity the system can displaywithout signal aliasing. The lower half of the color bar is ascale in shades of blue utilized to depict blood movingaway from the transducer. The number at the lower endof the velocity scale is equal to the number topping thescale, yet it is preceded by a negative sign implying oppositedirection. That number corresponds to the maximalvelocity of blood moving away from the probe that canbe detected without signal aliasing (6). As stated in theprevious section, the recommended aliasing velocity forcolor Doppler is between 40 and 70 cm/sec (20).Lowering the velocity scale (and therefore both positiveand negative aliasing velocities) below this range mayresult in color flow mapping that proves difficult to interpretbecause of excessive signal aliasing and mosaicflow. Moreover, by lowering the maximal velocity of thescale, every tint of red and blue represents a lower velocityinterval. Consequently, lower velocity blood flows areallocated brighter colors and become more obvious onthe display (6). Conversely, increasing the aliasing velocityabove the recommended range may result in colorjet areas appearing smaller: as the color scale is stretchedover a wider velocity range, the darker shades of red orblue are assigned higher velocities that consequentlybecome more difficult to see on the display.Shifting the baseline results in an asymmetrical velocityscale display. The maximal velocities at each end of thescale change, aliasing occurring at different velocitiesaccording to blood flow direction (6). For example, shiftingthe baseline upward results in a lower positive velocityat the top of the color Doppler scale and a more negativevelocity at the bottom of the scale display. Accordingly,color Doppler aliasing occurs at a lower velocity forblood flow directed towards the transducer and, on thecontrary, at a higher velocity in the case of blood flowmoving away from the transducer. Therefore, shiftingthe baseline may prove helpful to eliminate signal aliasingin color flow and PW Dopplers. It is also helpful in thequantitation of color flow jet (most often regurgitant jets)where the baseline of the color scale will be moved inthe direction of the flow to enable flow rate measurementby the proximal isovelocity surface area (PISA) method(see Chapter 5).E. Sector DepthColor jet size widens with increasing depth. Variousfactors contribute to this phenomenon. As previously discussed,the main determinant of the PRF is depth of field.Increasing the distance between the transducer and theregion of interest reduces the PRF and consequentlygenerates a larger color flow area. Another postulatedmechanism is the widening of the ultrasound beam inthe far field involving color Doppler mapping. Thesefactors contribute to produce a color jet that may artifactuallybe larger than the actual, anatomical far field structureconfining the flow (6,20).VI.SUMMARYSophisticated and accurate, TEE appears to grant the operator,a direct vision of a beating heart. One must rememberhowever, that this is not the case. The images are merely acomplex graphic reconstruction of the heart through mathematicalformulas and assumptions. Artifacts must be distinguishedfrom properly displayed structures, and normalvariants must not be confused with pathology. In order toavoid misinterpretations, appropriate instrument settingsare of paramount importance. Comprehension of themechanisms involved in artifact formation and familiaritywith the technique are prerequisites before performing andinterpreting transesophageal echoes. Finally, the operatormust stay alert and take the time to explore unusual findingsusing all the necessary imaging planes and modalities.Only then, will TEE achieve its full potential.REFERENCES1. Kremkau FW, Taylor KJ. Artifacts in ultrasound imaging.J Ultrasound Med 1986; 5(4):227–237.2. Feigenbaum Harvey. Echocardiography. Philadelphia: Lea& Febiger, 1994.3. Appelbe AF, Walker PG, Yeoh JK, Bonitatibus A,Yoganathan AP, Martin RP. Clinical significance andorigin of artifacts in transesophageal echocardiography ofthe thoracic aorta. J Am Coll Cardiol 1993; 21(3):754–760.4. Vignon P, Spencer KT, Rambaud G, Preux PM, Krauss D,Balasia B et al. Differential transesophageal echocardiographicdiagnosis between linear artifacts and intraluminalflap of aortic dissection or disruption. Chest 2001;119(6):1778–1790.5. Bach DS. Transesophageal echocardiographic (TEE)evaluation of prosthetic valves. Cardiol Clin 2000;18(4):751–771.6. Weyman AE. Principles and Practice of Echocardiography.2nd ed. Philadelphia: Lea & Febiger, 1994.7. Rao SR, Richardson SG, Simonetti J, Katz SE, Caldeira M,Pandian NG. Problems and pitfalls in the performance andinterpretation of color Doppler flow imaging: observationsbased on the influences of technical and physiologicalfactors on the color Doppler examination of mitral regurgitation.Echocardiography 1990; 7(6):747–762.8. Buttery B, Davison G. The ghost artifact. J Ultrasound Med1984; 3(2):49–52.9. Freeman WK, Seward JB, Khandheria BK, Tajik AJ. TransesophagealEchocardiography. Boston, MA: Little Brownand Company, 1994.