and in CCD, because green (only) is added to pulsatile flows, a format with which most users are familiar, the three-color mode is sometimes preferred. The two-color mode can, however, produce images which are more readi- ly understood by clinicians and patients, displaying arteries as red and veins as blue, as in an anatomy textbook. Resistive index is commonly used as an important para- meter in the evaluation of renal parenchymal diseases, acute obstruction, renal vascular diseases, renal masses, and transplanted kidneys (1 3). However, measuring RI is sometimes troublesome, especially when patients have dif- ficulty in breath-holding or in chronic renal disease in which the kidneys are small and show only slight vaculari- ty at CCD. Besides these patient factors, examiners who have little experience of renal Doppler US may have diffi- culty in selecting optimal arteries for RI measurement, re- sulting in unreliable RI readings and the lengthening of ex- amination time. Intrarenal arteries that are colored green at PFD can be good sites for obtaining clear spectral waveforms with a high Doppler signal and therefore for accurately measuring RI (Fig. 1). Use of the PFD function appears to facilitate the location of interlobar arteries optimal for RI measure- ment: examination time may be shortened by about 30 seconds, and CCD examination time may also be short- ened if followed by PFD examination. Although waveform quality showed no statistically sig- nificant difference between PFD and CCD examinations, 83.9% (47/56) of the waveforms in PFD were the same quality or better than those in CCD, a finding which indi- cates that by using the PFD function, RI examination times can be shortened without affecting quality. It might appear that within the overall time frame of a Doppler US examination, a difference of 30 seconds is triv- ial. However, shortening the examination time by using the PFD function may be quite helpful, especially when a patient’s condition does not allow adequate breath-holding for Doppler US. Kim PFD can also be a useful tool in organs other than the kidney. It can differentiate between hepatic arteries and portal veins, both of which flow in the same direction, and without being affected by flow direction, can display arteri- al flows in hypervascular tumors. Few articles describing the value of PFD have, however, been published (7). In conclusion, by demonstrating the optimal location for Doppler sampling in kidneys through the visualization of distinct color at CCD, PFD may be helpful in accurately and effectively measuring RI, and can shorten examination time without affecting the quality of the Doppler spectrum. Acknowledgments 1. This study was supported in part by the 2001 BK21 Project for Medicine, Dentistry and Pharmacy. 2. The author would like to thank GE Medical Systems, Korea, for their technical advice and support with refer- ences. References 1. Platt JF. Doppler ultrasound of the kidney. Semin Ultrasound CT MR 1997;18:22-32 2. Lee HJ, Kim SH, Jeong YK, Yeon KM. Doppler sonographic re- sistive index in obstructed kidneys. J Ultrasound Med 1996;15:613-618 3. Tublin ME, Dodd III GD. Sonography of renal transplantation. Radiol Clin North Am 1995;447-459 4. Keogan MT, Kliewer MA, Hertzberg BS, DeLong DM, Tupler RH, Carroll BA. Renal resistive indexes: variability in Doppler US measurement in a healthy population. Radiology 1996; 199:165-169 5. Eibenberger K, Schima H, Trubel W, Scherer R, Dock W, Grabenwoger F. Intrarenal Doppler ultrasonography: which vessel should be investigated? J Ultrasound Med 1995;14:451- 455 6. Seong CK, Kim SH, Sim JS. Detection of segmental branch re- nal artery stenosis by Doppler US: A Case Report. Korean Journal of Radiology 2001;2:57-60 7. Mori H, Haneki H, Yamakawa T, et al. Pulsatile flow detection for evaluation of tumor vessels of hepatocellular carcinoma. Nippon Shokakibyo Gakkai Zasshi 2000;97:484 48 Korean J Radiol 3(1), March 2002