Image Acquisition

All patients underwent comprehensive 2D and Doppler transthoracic echocardiography in the left lateral decubitus position using the Philips iE33 echocardiographic system (Philips Medical Systems, Andover, MA) with an S5-1 transducer (frequency bandwidth, 1–5 MHz). A standard clinical protocol for all examinations was followed in conjunction with American Society of Echocardiography guidelines for chamber quantification.

A magnetic localizer was attached to the S5-1 transducer by a molded plastic sheath. The magnetic localizer was connected to a dedicated console, from which a mechanical arm with an attached magnetic field generator hung over the patient ( Figure 1 ; VentriPoint Diagnostics Ltd, Seattle, WA). The localizer mounted on the ultrasound transducer detects orthogonal magnetic fields from the generator hanging over the patient, and in this manner the ultrasound probe position is localized in 3D space at the point of any 2D acquisition. A cushioned wedge was placed on the echocardiography couch to ensure that the metallic couch apparatus did not interfere with the magnetic field, and patients were instructed to remain entirely stationary in the left lateral decubitus position for the duration of a study acquisition. The ultrasound depth required to visualize all relevant structures was determined before commencing the study and remained fixed throughout.

Two-dimensional KBR apparatus: a localizing transducer (A) attached by a molded plastic sheath to a conventional 2D echocardiographic probe (B) detects orthogonal magnetic fields emitted by the generator (C) attached to the mechanical arm that hangs over the patient. Here in the 2D KBR calibration module, the die on the screen (D) represents the 2D echocardiographic probe, which moves synchronously with any movement of the 2D echocardiographic probe. Note the cushioned wedge in the background that is placed on the echocardiography couch to ensure that the metallic apparatus underneath couch does not interfere with detection of the magnetic fields.

Seven 2D transthoracic echocardiographic views were obtained in all subjects: parasternal long axis, parasternal short axis at the papillary muscle and apical levels, parasternal RV inflow, parasternal RV outflow including pulmonary valve hinge points and infundibulum, apical four chamber, and an off-axis RV apical view. The 2D KBR acquisition from each view consists of a 2-sec period (usually containing two or three heartbeats) acquired during end-expiratory breath-holds. The electrocardiograph was connected to the echocardiographic system via the dedicated 2D KBR console, and the console images were reproduced from the echocardiographic system’s video output and digitized at 30 frames/sec. Image quality was subjectively graded on a 5-point scale from 0 (very poor) to 4 (perfect).

Postprocessing: RV FAC

End-diastolic and end-systolic frames were assigned by visual identification of the largest and smallest RV four-chamber cavity areas, respectively, on the 2D KBR console. These frames were exported to the open-source OsiriX Digital Imaging and Communications in Medicine software for the measurement of RV FAC by tracing the RV endocardium in both frames and using the formula [(end-diastolic area − end-systolic area)/end-diastolic area] × 100.

Postprocessing: 2D KBR

The largest and smallest RV four-chamber cavity areas were visually identified as end-diastole and end-systole, respectively, on the 2D KBR console, with the software subsequently assigning the same time interval between these frames to all other views. On the 2D KBR console, a series of anatomic RV landmarks were identified on the 2D echocardiography images ( Figures 2A–2G ) in the end-diastolic frames and subsequently in the end-systolic frames. A minimum of 26 points was plotted for each of the end-diastolic and end-systolic data sets. RV endocardial points were placed at the junction between trabeculations and myocardium. The plotted anatomic landmarks with their respective 3D spatial coordinates were then submitted via the Internet to a secure remote server for remote processing by a proprietary 2D KBR algorithm. The algorithm interpolates between the plotted points by referencing against a catalogue of RV shapes generated by CMRI from patients with known diagnoses of PH.

Postprocessed 2D KBR data from a participant with pulmonary hypertension. All of the required 2D echocardiographic scan planes in end-diastole are displayed: (A) parasternal long-axis (PLAX), (B) PLAX RV inflow, (C) PLAX RV outflow including infundibulum and pulmonary valve hinge points, (D) parasternal short-axis (PSAX) at midcavity (papillary muscle) level, (E) PSAX apical level, (F) four-chamber RV, (G) off-axis RV apical view (note how the RV apex rides over the left ventricular apex). The differently colored cross-hairs represent user-defined plots for different RV structures; for example, red crosses are plotted along the RV endocardium, turquoise crosses along the RV side of the interventricular septum, a yellow cross at the RV apex, orange crosses at the pulmonary valve annulus, and purple crosses at the tricuspid valve annulus. The yellow border tracings are superimposed projections of the 2D KBR RV reconstruction onto the original 2D echocardiographic scan data, also showing how the polygon extends beyond the original 2D echocardiographic image sector. Landmarks can be checked and repositioned by the user if required, and the 2D KBR algorithm subsequently rerun. A final check is the nested view (H) of end-diastolic and end-systolic polygons to ensure alignment of the tricuspid and pulmonary valve orifices.

End-diastolic and end-systolic 3D models of the right ventricle were reviewed in a systematic fashion. Intersections between the borders of the 3D model and the original 2D scan plane were inspected to ensure concordance between 2D images and 3D reconstructions ( Figures 2A–2G ), and marked points were checked for alignment with the surface of the 3D model. Where significant deviations between the reconstructed model and either the plotted points and/or 2D echocardiographic endocardial borders existed, points were replotted and the algorithm was rerun. Where significant border versus 2D image misalignment suggested a shift in patient position or an inadequate breath-hold, all points from that 2D view were removed, and the erroneous 2D echocardiographic view was excluded from the 2D KBR reconstruction. A maximum of one view of the seven required in the data acquisition protocol could be excluded for any given study because of a change in patient position or an inadequate breath-hold. If this problem was encountered in more than one of the seven required views, the entire study was excluded from the final analysis.

The final check entailed inspection of the nested view of end-diastolic and end-systolic models to verify alignment of the tricuspid and pulmonary annular planes ( Figure 2 H). The final 2D KBR polygon was assessed for precision by subjectively scoring on a 5-point scale depending on the proximity of intersections of the plotted landmarks with the reconstructed polygon: 4 (all points intersect), 3 (three or fewer points significantly deviate from polygon), 2 (five or fewer points significantly deviate from polygon), 1 (seven or fewer points significantly deviate from polygon), and 0 (poor agreement).

FAC and 2D KBR Test-Retest Reproducibility

All subjects underwent serial 2D echocardiographic acquisition and postprocessing by two independent sonographers (D.S.K. and J.P.S.), as described previously. The two sonographers had similar experience in 2D transthoracic echocardiography (>4 years each) and received the same vendor training for the 2D KBR system. Sonographer 1 (D.S.K.) obtained a 2D KBR data set, after which sonographer 2 (J.P.S.) independently obtained a 2D KBR data set. Sonographer 1 then acquired a second 2D KBR data set. The sonographers, who were blinded to each other’s results and the results from CMRI, performed postprocessing of their own data sets for FAC and 2D KBR. Data sets analyzed for intraobserver test-retest reproducibility were postprocessed separately at time intervals of >2 weeks.

Image Acquisition

All CMRI images were acquired using a 1.5-T magnetic resonance scanner (Avanto; Siemens Healthcare, Erlangen, Germany) using a 12-element phased-array coil for signal reception and the body coil for signal transmission. A vector electrocardiographic system was used for cardiac gating. In all patients, ventricular volumes and great vessel flow were measured as previously described. Volumetric RV data were obtained using real-time radial k – t sensitivity-encoded imaging of contiguous transaxial slices. Real-time radial k – t sensitivity-encoded imaging allows the collection of high–spatiotemporal resolution, real-time images during free breathing and is part of the standard clinical CMRI work flow at our institution in the pediatric PH population.

Postprocessing

All image postprocessing was performed using “in-house” plugins for the open-source OsiriX Digital Imaging and Communications in Medicine software. Endocardial RV borders were traced manually at end-diastole and end-systole, the time points of which were identified by the largest and smallest RV cavity areas, respectively. The inclusion of RV trabeculations was the same as that performed in echocardiographic postprocessing. Ventricular stroke volume was the difference between the end-diastolic volume and end-systolic volume, and ejection fraction was calculated as (stroke volume/end-diastolic volume) × 100.