Study Design

The institutional review board of the University of Southern California approved the study protocol. All study participants provided written informed consent. A single portable ultrasound system dedicated to CIMT and plaque analysis, CardioHealth Station (CHS; CardioNexus Corporation, Houston, TX) was used. Training was based on the scanning protocol for imaging acquisition recommended by the ASE.

The study consisted of two prospective phases. The first phase included 60 subjects without history of cardiovascular disease (CVD) who were admitted for noncardiac reasons from August 2011 to November 2011 at a single tertiary medical center. All subjects underwent carotid imaging using CHS for CIMT measurement and carotid plaque assessment at a single encounter by both an expert (T.Z.N.) and R1 (K.D.). R1 initially underwent a training protocol developed by the expert for CIMT and plaque image acquisition and interpretation ( Table 1 ). Interobserver variability was determined and used to modify the original training protocol for the second medical resident (R2). A new modified training protocol ( Table 1 ), with an increased focus on plaque assessment, was implemented during a second phase in which both the expert and R2 (T.O.) scanned 10 new subjects (meeting the same clinical inclusion criteria as above). Each of these 10 subjects from phase 2 was scanned twice by both R2 and expert (1–24 hours apart). Interobserver and intraobserver CIMT and plaque variability using the modified training protocol was evaluated. Of note, both R1 and R2 used the same five mock subjects during training (phases 1 and 2) to establish baseline post-training interobserver variability between trainees. The expert scanner was a cardiologist and a core lab–trained IMT scanner with >8 years of experience in performing CIMT measurement and plaque assessment for randomized multicenter CIMT studies as well for clinical practice–implemented CIMT studies. Both R1 and R2 were third-year internal medicine residents without any prior experience in ultrasound imaging, CIMT imaging, or reading. The order of scanning between expert and R1 and R2 scanners was changed after every subject.

Resident Training

The training protocol is shown in Table 1 . Resident training included didactic and practical training sessions. Didactic sessions included online CIMT lectures, distribution and review of the ASE consensus article on CIMT, ultrasound principles, and image interpretation. Practical training sessions started with training residents on the features of the CHS system, followed by supervised and then unsupervised practical training sessions with the expert, as shown in Table 1 . Phase 2 training included the addition of dedicated time (5–15 min per subject) toward practicing short-axis and longitudinal circumferential sweep techniques for plaque assessment. This included a more focused and slower speed of carotid short-axis sweep, taking time to demonstrate carotid bulb and proximal internal carotid artery as well as the teaching of techniques to better differentiate plaque and artifacts (including the use of power Doppler), transducer manipulation including adjustment of probe pressure, probe angle, patient maneuvers, and demonstration of plaques in both short and one or more long axes. The distal 1 to 2 cm of common carotid artery (CCA) alone was imaged for CIMT because this appears to be representative of all segments, and all CCA, bulb, and internal carotid artery segments were imaged for plaque.

Inclusion of subjects into each phase of the study began after the expert scanner confirmed that each medical resident was capable of acquiring frozen images with adequate visual quality for CIMT measurement and plaque assessment.

Carotid Artery Scanning Technique

Scans were performed using the CHS system with a broadband 13-5V1/13-5 MHz linear-array transducer with a center frequency of 9 MHz, using B-mode display. This system displays raw radiofrequency data rather than digitally processed data displayed by the conventional ultrasound systems and has a theoretical resolution of approximately 30 μm. In phase 1 of the study, only a single-angle CIMT measurement was available on the CHS, and averaged three-angle measurement was available in phase 2. We used the single-angle mean CIMT measurement that displayed the thickest CIMT throughout phases 1 and 2 of the study. The CHS system determines systole and diastole on the basis of vessel luminal diameter allowing CIMT measurement at end-diastole without requiring external electrocardiography. The transducer was moved to align the flow divider with an on-screen vertical marker to ensure that the region of interest was automatically located at the far wall of the CCA (1 cm from the flow divider; Figure 1 ). Within the 1-cm region of interest, the system continuously tracks 24 spatial measurements at 200 frames/sec. Only the IMT data that are present at end-diastole are kept, and all nondiastolic frame IMT data are discarded, and a frozen image of tracked CIMT with measurements is then displayed. The CHS system incorporates a sensor in the transducer that tracks the scanning angle in real time and records the transducer angle on the screen along with the IMT measurement. Online manual CIMT border correction by manual tracing was also available in the setting of poor automatic border detection and was used when needed. Current ASE guidelines were followed. Head positioning was approximated to 45° to the right or left. The bilateral CCA was imaged in each patient both by the residents and the expert. For R1, plaque assessment was performed only in the live short-axis views acquired from the base of the neck to the highest segment that could be imaged above the carotid bifurcation, and long-axis assessment of plaque was performed only if plaque was seen in the short-axis view. For R2, plaque assessment was performed both in the short-axis view as well as in the long-axis sweep from the anterior to the posterior portion of the neck in all patients. CCA far wall mean CIMT was obtained by a circumferential (anterior to posterior) longitudinal at a single angle sweep along the CCA until the best horizontal single image with clear visualization of far wall lumen-intima and media-adventitia borders was obtained. A carotid plaque was defined as a focal intraluminal protrusion >50% of the surrounding IMT and with total thickness of ≥1.5 mm. A final report indicating mean right and left CCA far wall CIMT values and the presence and location of plaque, along with still images, was immediately available at the conclusion of each scan. The total time needed for manual scan completion was recorded for both the expert and novice scanners for all subjects.

Representative examples of normal ( top ) and thickened ( bottom ) intima-media are shown. An on-screen vertical marker ( top , white arrow ) is aligned with the flow divider (bifurcation point of bulb into the internal carotid artery and external carotid artery), and the region of interest is automatically located in the far-wall of the CCA (1 cm from the flow divider) along with an automated CIMT measurement as displayed.

Statistical Analysis

Statistical analysis was performed by a single blinded observer using Excel (Microsoft Corporation, Redmond, WA). Descriptive data are presented as mean ± SD, along with the coefficient of variation (CV). CIMT variability (intraobserver and interobserver) was analyzed using the “two one-sided t test” (TOST; 0.110-mm limits) and Bland-Altman analysis to determine both equivalency and level of agreement, respectively. Logistical regression analysis was performed to assess correlation (Pearson’s correlation coefficient). For noncontinuous data, specifically plaque comparisons, χ ² values were derived using SAS (SAS Institute Inc., Cary, NC). P values < .05 were considered statistically significant for all analyses. Although the CHS system has a theoretical axial resolution of approximately 30 μm (0.030 mm), we used the technological limits of standard ultrasound system resolution, approximating 0.1 mm when setting P _TOST limits (0.110 mm) as well as expected “good” CVs (average cohort CIMT, 0.550–0.650 mm; standard deviation, 0.500–0.100 mm by ultrasound limits; expected CV ≤ 15%–20%).

Study Population

Among the 60 subjects originally enrolled in phase 1, seven were excluded from final analysis because of the presence of initially unreported end-stage renal disease ( n = 3) or coronary artery disease ( n = 1), accidental capture of the jugular vein by R1 ( n = 1), an improperly saved image by R1 ( n = 1), and a single study with overall difficult image acquisition for both R1 and the expert ( n = 1), for a final phase 1 cohort of 53 subjects (106 vessels). From these 53 subjects, four more carotid vessels (right and left) were found to have nondiagnostic image quality by R1 for mean CIMT measurement analysis. The corresponding four diagnostic contra lateral vessels were also excluded for analysis purposes. One patient was noted to have concentrically thickened CIMT (>2.5 mm) and was included only for plaque analysis. The final phase 1 cohort included 96 vessels from 53 subjects for CIMT comparison and 98 vessels for plaque assessment. The frequency of nondiagnostic image acquisition was found to be <1% (one of 120) for the expert and <6% (seven of 120) for R1. The final phase 2 cohort included 10 separate subjects (each scanned twice by both the expert and R2) with 40 vessels for CIMT comparison and 20 vessels for plaque assessment. There were no subject or vessels excluded from phase 2. The expert and R1 or R2 each performed their scans first approximately 50% of the time within each respective phase cohort.

Clinical Characteristics

Clinical characteristics are summarized in Table 2 . Our subject cohort was at low CVD risk per Framingham risk score (mean, 6.5%). The average age of our final full cohort (total n = 63) was 45 ± 13 years (range, 25–65 years). There were 42 men, 45 Hispanics (71%), five African-Americans, six Asians, and seven Caucasians. Approximately one third each had histories of hypertension (29%) and diabetes mellitus (35%). A small number had significant or active histories of smoking (14%). The mean Framingham risk score in those with plaque versus without plaque (by the expert scans) was 9.04 ± 7.22 versus 5.11 ± 5.24 ( P < .01).

Table 2

Clinical characteristics of the study population

Variable	Value
Total population	63 (100%)
Men	42 (67%)
Women	21 (33%)
Total population age (y)	45 ± 13
Male age (y)	45 ± 11
Female age (y)	45 ± 15
Hispanic	45 (71%)
African American	5 (8%)
Caucasian	7 (11%)
Asian	6 (10%)
Hypertension	19 (29%)
Diabetes mellitus	22 (35%)
Smoking history	9 (14%)
Total cholesterol (mg/dL)	159 ± 60
Low-density lipoprotein cholesterol (mg/dL)	96 ± 36
High-density lipoprotein cholesterol (mg/dL)	43 ± 20
Glycosylated hemoglobin (%)	7.0 ± 2.7
Framingham risk score (%)	6.5 ± 5.9

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