Short-axis Orientation Research Articles

Computerised planning of the acquisition of cardiac MR images

A method to automatically plan acquisition of magnetic resonance images aligned with the cardiac axes is presented. Localiser images are acquired with a mean short axis orientation calculated from a group of ( n=50) adult patients. These images are segmented using the expectation maximisation algorithm. The borders of the ventricular blood pools are found and used to provide an estimate of the orientation of the cardiac axes. These estimated orientations are compared with corresponding manually aligned orientations. The method has been tested on n=12 volunteers showing an error of within 12° which is sufficiently accurate for clinical use.

Cine MRI using steady state free precession in the radial long axis orientation is a fast accurate method for obtaining volumetric data of the left ventricle.

In this study we assessed the use of a steady state free precession (SSFP) cine sequence in a series of radially orientated long axis slices for the measurement of left ventricular volumes and mass. We validated the radial long axis approach in phantoms and ex vivo porcine hearts and applied it to normal volunteers and patients using the SSFP and turbo gradient-echo (TGE) sequences. High quality images were obtained for analysis, and the measured volumes with radial long axis SSFP sequence correlated well with short axis TGE and SSFP volumes (r > 0.98). The best interobserver agreement for left ventricular volumes was obtained using SSFP in the long axis radial orientation (variability < 2.3%). We conclude that this combination of sequence and scan orientation has intrinsic advantages for image analysis due to the improved contrast and the avoidance of errors associated with the basal slice in the short axis orientation.

Real-time cardiac cine imaging with SPIDER: steady-state projection imaging with dynamic echo-train readout.

Steady-state projection imaging with dynamic echo-train readout (SPIDER) is a multiecho radial k-space trajectory TrueFISP sequence developed for real-time cine imaging of the heart. This new pulse sequence combines the superior SNR and blood-to-myocardium contrast of TrueFISP with the increased scan time efficiency of EPI and undersampled projection reconstruction. SPIDER sequence RF repetition time (TR) was minimized by limiting the echo-train to a length of three while acquiring the first and third echoes asymmetrically. A temporal resolution of 45 ms was achieved with TR/TE1/TE2/TE3 of 3.24/0.6/1.6/2.6 ms and a factor of 2 view sharing scheme. Phantom experiments showed little difference between the weighting of the signals acquired at each of the echo times but did show considerable off-resonance modulation between them. In vivo experiments demonstrated the feasibility of using the SPIDER sequence for real-time imaging in the cardiac short axis orientation.

Rapid evaluation of left ventricular volume and mass without breath-holding using real-time interactive cardiac magnetic resonance imaging system

OBJECTIVES The purpose of this study was to validate cardiac measurements derived from real-time cardiac magnetic resonance imaging (MRI) as compared with well-validated conventional cine MRI. BACKGROUND Although cardiac MRI provides accurate assessment of left ventricular (LV) volume and mass, most techniques have been relatively slow and required electrocardiogram (ECG) gating over many heart beats. A newly developed real-time MRI system allows continuous real-time dynamic acquisition and display without cardiac gating or breath-holding. METHODS Fourteen healthy volunteers and nine patients with heart failure underwent real-time and cine MRI in the standard short-axis orientation with a 1.5T MRI scanner. Nonbreath-holding cine MRI was performed with ECG gating and respiratory compensation. Left ventricular end-diastolic volume (LVEDV), left ventricular endsystolic volume (LVESV), ejection fraction (EF) and LV mass calculated from the images obtained by real-time MRI were compared to those obtained by cine MRI. RESULTS The total study time including localization for real-time MRI was significantly shorter than cine MRI (8.6 ± 2.3 vs. 24.7 ± 3.5 min, p < 0.001). Both imaging techniques yielded good quality images allowing cardiac measurements. The measurements of LVEDV, LVESV, EF and LV mass obtained with real-time MRI showed close correlation with those obtained with cine MRI (LVEDV: r = 0.985, p < 0.001; LVESV: r = 0.994, p < 0.001; EF: r = 0.975, p < 0.001; LV mass: r = 0.977, p < 0.001). CONCLUSIONS Real-time MRI provides accurate measurements of LV volume and mass in a time-efficient manner with respect to image acquisition.

A model of displacement and strain for arc-shaped mountain belts applied to the Jura arc

A plan view geometric model for simple, parallel, differential displacements is presented. As an analogue for models of arcuate mountain belt formation we use the model to predict strain patterns produced by parallel displacement in front of a rigid versus deformable indenter. A rigid indenter is simulated by an irregular quadrilateral displaced a constant amount along its hinterland boundary. A deformable indenter is simulated by an irregular quadrilateral displaced along a hinterland boundary that is allowed to rotate. Some simple test cases show that the deformable indenter model leads to a pattern of strain very similar to that encountered in arcuate mountain belts. Short axes orientations are most deviated from the transport direction along the lateral edges of the model with minimum displacement and shortening and vary across the central domain, mirroring strain features with variable orientations from mountain belts such as fold trends and minor deformation features such as horizontal stylolite peaks. The rigid indenter model also generates short axes deviated from the transport direction but showing far less variation. Displacement–strain relationships from the Jura mountains (Switzerland and France) are quantified based upon a simplified version of the finite displacement field for the Jura fold–thrust belt of Philippe, Y. (1995) [“Rampes latérales et zones de transfer dans le chaines plissées”. (Unpublished PhD thesis, Université de Savoie)]. We find that the model short axis orientation pattern is very similar over at least the eastern and central Jura to the stylolite patterns from the region whilst the long axes closely match the fold axes trends. The model suggests that the Jura mountains could have formed as a result of a progressive deformation with uniform transport in a general northwest direction.

Assessment of regional left ventricular wall parameters from short axis magnetic resonance imaging using a three-dimensional extension to the improved centerline method.

Short-axis magnetic resonance images of the cardiac left ventricle, acquired in multiple slices and phases, may be used for the quantitative assessment of regional wall parameters. Conventional two-dimensional (2-D) methods for wall thickness measurement rely on information within one imaging plane, which may result in overestimation of the true thickness depending on the local direction of myocardial wall with respect to the imaging plane. To perform wall thickness measurements truly perpendicular to the myocardial wall, a three-dimensional (3-D) wall thickness calculation algorithm has been developed based on the 2-D improved centerline method. An evaluation was performed on left ventricular-shaped software phantoms, and on the magnetic resonance (MR) imaging data obtained from 20 healthy individuals. The 3-D method applied to software phantoms with an angulation within 20 degrees of the true short-axis orientation demonstrated only a 1.6% overestimation of wall thickness at the mid to low slices, and a 10.6% error at the apex (2-D measurements: 8.1% and 28.6%, respectively). Three-dimensionally calculated wall thickness in the healthy individuals was systematically and significantly smaller than corresponding 2-D wall thickness (by 11.2%, 8.7%, and 2.6% at the apical, low, and mid slices, respectively). Cardiac wall thickness measurements from short-axis MR studies can be obtained with a higher accuracy by the newly developed 3-D approach than with the conventional 2-D approach.

Global cardiac function using fast breath-hold MRI: validation of new acquisition and analysis techniques.

Calculation of global cardiac function parameters has been validated using fast, segmented k-space, breath-hold, gradient-echo, magnetic resonance images. Images of phantoms, experimental animals, normal volunteers, and patients were acquired with a 1.5 T clinical scanner. Humans were imaged using two phased-array surface coils in multicoil mode. Myocardial contours were extracted using a new interactive, semi-automated method based on the active contour model method. Images were acquired in the short-axis orientation, and, using a new imaging and analysis strategy, in rotating plane long-axis orientations, to provide better definition of the valve planes and the apex, and also to reduce the number of slices (compared with the short-axis method) required to sample the whole heart. Validation was accomplished through calculation of the volumes of phantoms and left and right ventricular masses of animal hearts. Functional parameters from MRI were compared with those from echocardiograms and radionuclide angiograms in normal volunteers and patients, respectively.

Noninvasive quantification of left ventricular myocardial mass by gated proton nuclear magnetic resonance imaging

The validity of cardiac nuclear magnetic resonance imaging for determination of left ventricular myocardial mass was evaluated in nine dogs. A gated spin echo-pulsing sequence was used for in vivo imaging, obtaining 0.7 cm thick slices of the heart spaced by 1 cm. On each imaged slice, the left ventricular surface area was reproducibly determined by planimetry and was multiplied by slice spacing and specific gravity of the myocardium (1.05) to obtain slice mass. Total left ventricular mass was calculated by adding slice masses in short-axis (method I), transaxial (method II) and vertical long-axis (method III) orientations using Simpson's rule. With each method, masses of the portions of the left ventricle subject to partial volume effect either were not accounted for or alternatively were estimated from the same or an orthogonal imaging plane. Calculated left ventricular mass was compared with the actual excised left ventricular weight. With NMR imaging of in situ nonbeating hearts, best results were obtained when either method I or method II was used and partial volume effect was estimated either from the same or an orthogonal plane. With in vivo NMR imaging, best results were noted when method I was used and mass of the partial volume apex was calculated from transaxial slices: Y (in vivo NMR image) = 8.3 + 0.99X, r = 0.996, SEE = 3.14. For this method, the interobserver reliability coefficient and standard error of the measurement were 0.97 and 5.4, respectively. Compared with method I, in vivo methods II and III were associated with larger errors (SEE ranging from 13.03 to 19.03) regardless of the approach used to estimate partial volume effect. It is concluded that NMR imaging is a highly accurate noninvasive method for in vivo measurement of left ventricular mass in dogs and offers promise for accurate measurement of left ventricular mass in patients.

Gated cardiac magnetic resonance structural imaging: optimization by electronic axial rotation.

Most magnetic resonance imaging has used body orthogonal axes with the Z axis placed along the length of the body and the X and Y axes at right angles to the body. This orientation is not optimum for the heart; visualization of sections along the short and long cardiac axes would best define cardiac structural detail and functional status. The new orientation was accomplished by selection of electronic angulation of the magnetic fields for each subject rather than by attempting to approximate the cardiac axes by altering the position of the patient. This technique improved visualization of comparative wall segments, valvular structures, and the true four-chamber view of the heart, and also gave the best visualization of the pericardium. In addition, more accurate estimates of chamber size and myocardial mass can be made from the short-axis orientation, since the sections are orthogonal to the myocardium.