Functional and Morphological Cardiac Magnetic Resonance Imaging of Mice Using a Cryogenic Quadrature Radiofrequency Coil

<div><p>Cardiac morphology and function assessment by magnetic resonance imaging is of increasing interest for a variety of mouse models in pre-clinical cardiac research, such as myocardial infarction models or myocardial injury/remodeling in genetically or pharmacologically induced hypertension. Signal-to-noise ratio (SNR) constraints, however, limit image quality and blood myocardium delineation, which crucially depend on high spatial resolution. Significant gains in SNR with a cryogenically cooled RF probe have been shown for mouse brain MRI, yet the potential of applying cryogenic RF coils for cardiac MR (CMR) in mice is, as of yet, untapped. This study examines the feasibility and potential benefits of CMR in mice employing a 400 MHz cryogenic RF surface coil, compared with a conventional mouse heart coil array operating at room temperature. The cryogenic RF coil affords SNR gains of 3.0 to 5.0 versus the conventional approach and hence enables an enhanced spatial resolution. This markedly improved image quality – by better deliniation of myocardial borders and enhanced depiction of papillary muscles and trabeculae – and facilitated a more accurate cardiac chamber quantification, due to reduced intraobserver variability. In summary the use of a cryogenically cooled RF probe represents a valuable means of enhancing the capabilities of CMR of mice.</p> </div

End-diastole (a) and end-systole (b) short axis views acquired using the CryoProbe together with a ultra-high spatial resolution of (43×138×300) µm³.

<p>Endo- and epicardial border delineation as well as the depiction of anatomic details of the papillary muscles and trabeculae are improved versus the high resolution CryoProbe images (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0042383#pone-0042383-g003" target="_blank">Figure 3</a> e,f) due to the enhanced spatial resolution used in the ultra-high spatial resolution protocol.</p

Box plot of relative brain vascular blood flow.

<p>Vascular area was calculated from two-dimensional section of angiographs of rat brains by grayscale analysis as described in the methods section. Relative vascular circulation of the individual animal is expressed as ratios of vascular areas from the last and the first TOF-MRA. Time between measurements was eight months. Ratios <1 indicate attenuated vascular blood flow. Data are given of animals treated with intravenous injections of the antibody to the α₁-AR (AB; n = 9), their controls (C-AB); n = 10), and rats injected subcutaneously with the α₁-AR peptide (PEP; n = 10) and the respective controls (C-PEP; n = 10). Asterisk indicates statistically significant differences with p<0.05.</p

Comparison of relative vascular areas.

<p>Cohorts of rats were treated either with intravenous injections of α₁-adrenoceptor antibody (AB) or with subcutaneous injections with α₁-adrenoceptor peptide (PEP). Control animals received unspecific control IgG (C-AB) or albumin (C-PEP). Applications of substances were repeated monthly. The time span between TOF-MRA measurement 1 and measurement 2 was eight months. Relative vascular areas were determined by grayscale analysis of two-dimensional TOF-MRA maximum-intensity-projections. Values are given as means ± standard error of the mean. The number of animals of each group is given in brackets.</p>*<p>significantly different to measurement 1 (p<0.05).</p

Comparison of end-diastole (left column) and end-systole (right column) short axis views acquired using a spatial resolution of 156×234×800 µm³ (a,b) and 69×115×800 µm³ (c,d,e,f).

<p>The depiction of anatomic details for left ventricular papillary muscles and right ventricular trabeculae is enhanced in the CryoProbe (CP) images (e,f) compared to the room temperature (RT) coil images (a,b,c,d) for both diastole and systole. Coronary arteries are more pronounced in the CryoProbe images. The lateral right endocardial boundary is better delineated in the CryoProbe images. The signal-to-noise ratio of the lower resolved images acquired with the RT-coil (47±9, a,b) is comparable to that of the higher resolved images acqured with the CryoProbe (54±7, e,f).</p

<i>a,b:</i> Bar plot of mean LV myocardium signal-to-noise ratio (SNR) (b) measured in images acquired with the CryoProbe (CP) or room temperature coil (RT) for all mice in the different segments of a six-segment model (a).

<p>The segments were numbered clockwise, starting at the inferoseptal segment. The region closest to the coil (segment 3) showed the highest SNR, and SNR decreased with distance from the coil. <i>c,d</i>: Bar plot of mean LV myocardium SNR (d) for all mice in the different slices (geometry shown in c). Slices were numbered from the basal slice to the apex. For the CryoProbe the apical slice showed an unexpected increase in SNR, in contrast to the decreasing trend from base to apex in the remaining 6 slices.</p

Two-dimensional maximum-intensity-projections of

<p><b>TOF-MRA of rat brain.</b> Coronal images of rat brains were screened in 3 mm slices. The rat shown in A, B was immunized with the α₁-AR peptide, the rat shown in C, D obtained vehicle as control. Injections were performed monthly. Angiographs of the same animal were taken at the beginning of the treatment (A, C) and eight months later (B, D). Light contrasts correlate with vascular blood flow. Eight months of treatment with the α₁-AR peptide led to apparent attenuations of blood flow (B, right part of brain section) compared to the initial situation (A). Control injections did not result in comparable defects in the same time frame (D versus C).</p

Black and white threshold images.

<p>Two-dimensional sections of maximum-intensity-projections (MIP) of TOF-MRA images were processed as described in Materials and Methods. The black and white images were generated using the median plus two standard deviations as threshold value. White areas correspond approximately to the area of vessels with blood flow. Exemplarily shown is one animal treated with intravenous injections of the α₁-AR antibody (A, B) and one control animal (C, D) at measurement 1 (A, C) and at measurement 2 eight months later (B, D). The reduction of the white areas in B compared to A is evident.</p

Analysis of rat sera for the presence of antibodies to the α₁-AR.

<p>Antibodies were detected by ELISA. (PEP) rats immunized with the α₁-AR peptide, (C-PEP) controls treated with vehicle. Blood samples were taken about one month before the final TOF-MRA. The obtained sera were analyzed at a 1∶50 dilution.</p