Carotid intima-media thickness measurements are not affected by the ultrasound frequency

Objective: Carotid intima-media thickness (IMT) is a B-mode ultrasound measure of subclinical atherosclerosis predictive of future cardiovascular risk. Carotid IMT measurements were historically obtained at an ultrasound frequency of 8 MHz or lower, but it is unknown whether measurements obtained at higher frequencies using newer, more advanced ultrasound technology allow for valid comparison to the older general population databases that are commonly used for the interpretation of carotid IMT results. Methods: Carotid IMT studies were conducted in 35 consecutive patients at standard (8 MHz) and high (14 MHz) frequencies and measurements were performed by two independent expert readers. Systematic bias was assessed by using the paired t-test and agreement was analyzed with the Bland-Altman method. Results: The sample mean carotid IMT obtained at 14 MHz was 0.006 mm lower than that obtained at 8 MHz. The 95% confidence interval (CI) for the mean difference between frequencies indicated that the population mean for 14 MHz is unlikely to be more than 0.02 mm lower than for 8 MHz (95% CI -0.017 to 0.004). The 95% reference range for the difference between the two transducer frequencies indicated that the thickness obtained at 14 MHz was within 0.05 mm of that obtained at 8 MHz for 95% of subjects. Conclusions: Carotid IMT measurements obtained at higher transducer frequencies are similar to those obtained at standard frequency. This finding has important clinical implications because it validates comparison of carotid IMT measurements obtained with newer, more advanced ultrasound technology with the landmark reference carotid IMT studies commonly used for interpretation of carotid IMT results.