The main study objectives were: 1) to determine and compare metrics of left ventricular volumes and function between CMR and echo in a consecutive cohort of post-MI patients presenting for imaging evaluation of cardiac function, 2) to compare the sensitivity of the 2 modalities to assess regional wall motion (RWM) after MI, and 3) to quantitatively assess changes in these parameters after MI by a comparison of study patients with a cohort of normal volunteers.
Forty-seven study subjects were prospectively enrolled from a consecutive series of patients with MI occurring >6 wks previously referred for echo evaluation of cardiac function and who consented to study participation, which included undergoing the complementary CMR study within 1 hour of echo. The protocol was approved by the Western Institutional Review Board. Standard electrocardiogram (ECG) and serial cardiac biomarker analysis was used to document MI.
Each subject served as his/her own control. Predefined cardiovascular measurements were made for CMR and echo using workstation-specific methods by a seasoned observer blinded to results of the complementary imaging method. Imaging results and demographic information were entered into a study database.
Given limited information on the normal range for CMR measurements and dependence of work-station specific methodology, we determined a normal range for CMR metrics in a group of 30 local normal volunteers recruited concurrently . These subjects were consenting, uncompensated adults of either sex (equally divided between men and women) of ages between 40–60 years who underwent a standard CMR functional study and who had no clinically apparent cardiovascular diseases, including hypertension or diabetes on screening history, or abnormal physical on screening examination, or significant other-organ diseases. Echocardiographic normal values were taken from the published literature.
Cardiac Magnetic Resonance Imaging (MRI) and Echo Studies
Following axial and sagittal localizer sequences, standard cardiac 2, 3, and 4 chamber 1 cm thick long axis and short axis slices were obtained on a General Electric (GE) 1.5 Tesla magnet (EXCITE platform, version 11.0) using steady state free precession (SSFP) cine sequences (typical in-plane resolution 2.2 × 1.3 mm2). All images were acquired using a phased-array 8-channel cardiac coil during single breath-holds (maximum, 15 seconds) with ECG (preferred) or peripheral (finger pulse) triggering. Acquisition parameters were as follows: zoom mode, TR = 2.4 msec, TE = min full, flip angle = 45°, field of view = 35 × 35 cm, matrix = 192 × 160, NEX = 0.5, slice thickness = 8 mm with zero spacing, phases = 20, and views per segment ≤ 24 to maintain temporal resolution < 80 msec. No vasoactive agents were given between echocardiography and CMR imaging.
Echocardiographic images were obtained in the standard parasternal long and short axis and apical 4 chamber and 2 chamber views utilizing digital Vivid 7 ultrasound equipment with a combined tissue imaging 2.5 – 4.0 MHz transducer (GE, Milwaukee, WI). At least 3 cardiac cycles were captured at the left ventricular (LV) base, mid papillary muscle level, and apex for wall motion assessment. No intravenous echocardiographic contrast agent was used. Later, an expert reader obtained measurements off-line from the parasternal and apical windows blinded to patient identity, gender, and clinical data. Two dimensional (2D) echo ventricular volumes and LV ejection fraction were planimetered from the 4-chamber and 2-chamber areas using the modified Simpson's rule. All of the measurements were obtained in concordance with American Society of Echocardiography standards .
Off-line image post-processing was performed on a cardiac Delta workstation with ReportCard 2.0 software (GE, Waukesha, WI). Measurements followed standard CMR procedures or mimicked the approach to echo measurement where appropriate. Image grayscale was adjusted to maximize the myocardial blood pool contrast without maximal pixel intensity saturation. Manual planimetry was performed by an expert CMR technologist and independently confirmed by the CMR physician reader. (Observers were blinded to echo results.) In the short axis view, the most basal slice used for LV volume analysis excluded any LV outflow tract. Unattached papillary muscles were included within the left ventricular chamber. Volumes for CMR were determined from the stack of short axis slices using Simpson's rule and for echo using the modified, semi-Simpson's rule based on 4- and 2-chamber planimetry [1, 7, 8]. For both modalities, floating papillary muscles were included within the chamber volume measurements. For both modalities, wall segments were divided by consensus into 2 general categories: good/adequate and suboptimal/poor. To provide a conservative comparison of the 2 modalilties, poorly imaged segments were excluded from the comparative analyses.
Consensus quantitative results were entered prospectively into research databases, and computer-assisted manual tracings were saved for later visual comparisons. Similarly, echo image analysis was performed by experienced observers blinded to CMR study results and followed standard techniques [7–9].
Segmental wall motion analysis used a standard 17-segment model .
Study demographic variables included subject age, sex, heart rate, weight, height, and body surface area (BSA). Cardiovascular variables included LV end-diastolic (EDV) and end-systolic (ESV) volumes and the derived variables stroke volume (SV, SV = EDV-ESV), and LV ejection fraction (EF = SV/EDV). For RWM, the LV was divided into 17 segments , and each visualized segment was graded on a scale of 1 to 5 as normal, hypokinetic, akinetic, or dyskinetic, or aneurysmal, respectively. Wall motion then was assessed as a summed total score, a worst segmental score, and an average RWM score, calculated as the average of all scored segments .
Results are presented as mean (standard deviation). Pearson's correlation coefficient, paired t-tests, and chi-square tests were used to compare intra subject and inter modality CMR and echo metrics, as appropriate. SPSS for Windows (version 14.0, SPSS Inc., Chicago, IL) was used for statistical analysis. A p value of 0.05 or less was deemed nominally significant and 0.01 or less definitely significant, given the 5 primary outcome variables.