High resolution high field rodent cardiac imaging with flow enhancement suppression

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l Original Contribution

Magnetic Resonance Imaging, Vol. 12, No. 8, pp. 1183-1190, 1994 Copyright 0 1994 Elsevier Science Ltd Printed in the USA. All rights reserved

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HIGH RESOLUTION HIGH FIELD RODENT CARDIAC IMAGING WITH FLOW ENHANCEMENT SUPPRESSION

STEPHEN E. ROSE, STEPHEN J. WILSON, FERNANDO 0. ZELAYA, STUART CROZIER,

AND DAVID M. DODDRELL

Centre for Magnetic Resonance, University of Queensland, St. Lucia Qld 4072, Australia

A method that incorporates cardiorespiratory-gated 2DFT spin-echo imaging with blood flow enhancement suppression is described which enables high resolution microimaging of the rodent heart. This methodology was applied to obtain in vivo cardiac mouse and rat images with in-plane resolutions of 100-200 pm using high field vertical bore magnet systems. Suppression of intraventricular blood flow enhancement was achieved using a combined spin-echo/gradient-refocussed sequence to dephase magnetization from flowing spins prior to imaging.

Keywords: Rodent cardiac MR imaging; Cardiorespiratory triggered imaging; Blood flow enhancement compensation.

INTRODUCTION

Magnetic resonance imaging (MRI) is now a widely es- tablished diagnostic imaging modality in clinical radio- logical practice. Similarly, MR microscopy and to a lesser extent image guided localized magnetic resonance spectroscopy (MRS) are currently being extensively used to study human disease in laboratory animal mod- els, in vivo. In this field of research, most imaging studies have primarily targeted the rodent brain and abdomen, using either small or medium bore (~40 cm) horizontal magnet systems. I*’ Rodent thoracic imaging, on the other hand, has not been as extensively investi- gated,3-6 partially due to the additional requirement of suitable respiratory and cardiac gating hardware. Com- pared with human MR cardiology, constraints on rodent gating hardware are more severe because of the requirement of monitoring and gating heart rates up to 450 beats/min.

In our present study we have utilized a modified cardiorespiratory gated 2DFT spin-echo imaging technique coupled with blood flow enhancement suppression to obtain high resolution cardiac images in both the rat and mouse. All imaging experiments were performed on vertical bore magnet systems using custom

designed shielded gradient sets, birdcage resonator radio frequency (RF) probes and animal handling systems with cardiorespiratory gating capabilities.

Although fast scan methods employing ultra fast gradient echo techniques have been successfully used to assess rodent heart dynamic function,’ gated single and multislice spin-echo techniques were utilized in this study to ensure optimal spatial resolution and morphological information. A flow suppression sequence (see Fig. 1) comprising a combined spin-echo/gradient- refocussed preparation sequence (based on a similar scheme proposed by Nishimura et al8 for flow angiography) was modified to suppress signal from flowing blood. To our knowledge, this is the first reported application of the use of this sequence to minimize vascular and ventricular blood flow enhancement.

With this methodology, in vivo mouse heart images with in-plane resolutions of - 100 pm (transverse slices, Figs. 3A and 3B) and - 120 pm (sagittal/coronal slices, Figs. 4A and 4B) and rat heart images of -200 pm (transverse slices, Fig. 5A) were readily achievable. With respect to in-plane resolution, we believe the in vivo images presented in this study represent a signifi- cant improvement over previously reported rodent cardiac imaging results.

RECEIVED 11/15/93; ACCEPTED 6/16/94. Address correspondence to David M. Doddrell.

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1184 Magnetic Resonance Imaging 0 Volume 12, Number 8, 1994

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FLOW DEPHASINGJ m GRADIENT

Fig. 1. A combined spin-echo/gradient refocussed sequence for suppression of flow artifact.

METHODS

Imaging and Cardiorespiratory Triggering Hardware

Mouse and rat proton imaging studies were performed at 7 T (SWB, super wide bore, 150 mm bore) and 9.4 T (WB, wide bore, 89 mm) in vertical magnet systems. Both magnets were supplied by Bruker Spec- trospin (Fallanden, Switzerland) and interfaced to Bruker AMX consoles equipped for microimaging (Bruker Analytische Messtechnik GmbH, Germany).

RF probes and shielded gradient sets were designed and constructed at the Centre for Magnetic Resonance. The RF probes were 12 rung birdcage resonators’ with dimensions (dia. x length) of 38 x 30 mm (WB) and 64 x 60 mm (SWB). The resonator length-to-diameter ratio was chosen as a compromise between Br sensitiv- ity and homogeneity to allow for maximum spatial resolution for imaging in the sagittal and coronal planes. This ensured that folding artifact did not perturb image quality at the reduced field-of-view (FOV). The shielded gradients are based on a multilayer minimum inductance design. lo The gradients produced a maximum of 25 G/cm in the WB and 19 G/cm in the SWB set. In each case the shielded rms residual field at the bore tube was less than 1% of the unshielded rms field. Approximately 1% of pre-emphasis and Z. compensation were applied to cancel residual eddy currents.

Cardiorespiratory triggering was performed using bipolar ECG leads with Ag/AgCl electrodes (3M HealthCare, St. Paul, MN, USA) and an optical respiratory sensor. ” Both signals were displayed on a Physioguard SM 785 NMR Monitor (Bruker Medizin- technik, Germany). Trigger pulses and preset delays from the R wave peak and respiratory threshold were taken from the unit and combined in an external logic circuit (see Fig. 2A).

A trigger pulse (active low) is sent to the spectrometer on the detection of the first R wave after the respiratory threshold is crossed. The exact timing of this pulse is subject to preset delays within the monitor and within the pulse program. This is achieved in the cir-

cuit as shown (Fig. 2B). A flip-flop is set on crossing of the respiratory threshold and the output state is log- ically ANDed with the next R wave pulse. The resulting signal is conditioned for the spectrometer and resets the flip-flop for the next cycle. Thus one trigger pulse per respiratory cycle occurs at the first available systole.

Imaging Protocols Rodent thoracic images were acquired with either a

standard cardiorespiratory triggered 2DFT spin-echo technique or a composite flow suppression spin-echo sequence. The blood flow enhancement suppression sequence consisted of three nonselective RF pulses (Fig. l), with symmetrical flow dephasing gradients placed either side of the refocussing 180” pulse.8*‘2 A dephasing gradient strength of either 1.2 or 7.2 G/cm was applied in the slice direction for a total of 2 ms. With this sequence, magnetization of interest is in the transverse plane for 6 ms. The TE time for the spin- echo imaging experiment (without flow compensation) was normally set to 16 ms.

To avoid any saturation effects, a single application of the flow suppression sequence was applied immedi- ately prior to the multi-slice imaging segment. With this methodology, acquisition of up to eight slices of a single cardiac cycle could be achieved between consecutive QRS waves. For some rat thoracic imaging experiments, blood flow suppression was achieved using spatial- presaturation techniques as reported by Felmlee et a1.13 This consisted of two lo-mm presaturation slices positioned exterior to the image volume.

Contiguous images of mouse and rat heart were acquired with l-l.5 mm and 2 mm slice thicknesses, respectively. For thoracic mouse imaging, FOV of either 25 mm (transverse images) or 30 mm (sagittal/coronal slices) were employed. Transverse images of the rat thorax were acquired with a 48 mm FOV. The image matrix size was normally 256 x 256 with a single average acquisition. Depending on the animal’s respiratory rate the average imaging time was approximately 15-20 min.

Mouse microtome slices of 10 pm were prepared from a thoracic paraffin tissue block. Sections were stained with H&E to enhance contrast.

Animal Protocol Female Quarkenbush mice weighing 30-40 g and fe-

male Wistar rats weighing 130-150 g were induced with 3% isoflurane in O2 (3 l/mm). Rodents once fitted with paw ECG electrodes and abdominal respiratory sensor, were then placed in a head-up position in the magnet. The animals were maintained with 1.5-2% isoflurane in O2 (0.5-l l/min). Isoflurane was adminis- tered to the animals via a nose cone.

Rodent cardiac imaging 0 S.E. ROSE ET AL. 1185

ECG electrodes

0 --rkger out

Logic Unit

Resp Sensor

6 /i I Optical Fibre

Ventricular Diastole

Trigger to spectrometer

(4

Trigger to

u- Spectrometer

Monostable

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Fig. 2. An external logic diagram (A) for cardiorespiratory gating. (B) A combined ECG and respiratory gating circuit for spectrometer triggering. In (A), NX n data acquisitions occur between consecutive QRS waves, where Nand n denote the number of slices and averages, respectively. In addition, the phase of the cardiac cycle with respect to the triggering delay after the QRS peak is also presented.18

RESULTS

Shown in Fig. 3B is a cardiorespiratory-gated transverse image of the mouse heart captured during diastole. The image was acquired without flow enhancement suppression to enable identification of the descending aorta from other major vessels. In comparison to the 10 pm microtome thoracic slice at approximately equivalent image slice position (Fig. 3C), structures such as the oesophagus, branches of the pulmonary artery, and bronchi can be depicted. Intraventricular structures such as the right and left ventricle, including papillary muscles and mitral valve leaflets, can also be observed, along with a number of coronary arteries and veins in the ventricular walls and septum.

In addition to delineating ventricular structures, sagittal and coronal images (Figs. 4A and 4B) enable vi- sualisation of both left and right atria, with the aorta most readily identifiable in the coronal slice. In this case, both images were acquired with flow enhancement suppression. The ability to monitor both ventricles and atria in the single slice, highlights the importance of being able to image the heart in both the sagittal and coronal plane.

Transverse rat thoracic images acquired from a multi-slice experiment are shown in Figs. 5A and 5B. Orientation of the slice (Fig. 5B) is superior to the heart through the level of the thymus gland. Suppression of blood flow enhancement signal was achieved using presaturation slices positioned exterior to the imaging re-


(A)

(Cl

gion. At this level of the aortic arch both ascending and descending aorta are easily observable along with the superior vena cava and pulmonary artery.

DISCUSSION

Multi-slice cardiorespiratory gated spin-echo images of the rodent heart can be severely affected by hyperintense intraventricular blood signal. This hyperintense signal, which results from the partial refocussing of the

(B)

Fig. 3. (A) and (B) Cardiorespiratory gated 2DFT spin-echo images of the mouse heart captured during diastole. Images were acquired without any bloodflow-enhanced suppression. Spectrometer triggering occurred approximately 100 ms after the R wave. The images were obtained using a 9.4T, 89 mm vertical bore magnet system. Slice thickness, TE time, FOV, matrix size, and number of averages were 1.5 mm, 16 ms, 25 mm, 256 x 256, and 2, respectively. These parameters yielded an in-plane resolution of < 100 pm. (C) Microtome slice (10 pm) of a mouse thorax corresponding to a similar slice position as that in image (B). Abbreviations for (B) and (C): L.V., left ventricle; R.V., right ventricle; P.M., papillary muscle; if M.V., leaflets of mitral valve; Cor. A. & V., coronary arteries and veins; b of P.A., branches of pulmonary artery; D. Ar . , descending aorta; Oes, Oesophagus; Br . , Bronchi; Lu, Lung.

flowing blood magnetization during the spin-echo experiment, can lead to loss of morphological information. The extent to which blood signal appears in the ventricle is dependent on the phase of the cardiac cycle when captured during a multi-slice imaging sequence.14 This effect is clearly demonstrated in the multi-slice image array given in Fig. 6. The first image in the four- slice series (A) possesses extensive blood signal in the left ventricle. In this case, the multi-slice experiment was triggered 25 ms after the R wave resulting with the


W

Fig. 4. Blood flow enhancement suppressed sagittal (A) and coronal (B) images of the mouse thorax acquired approximately 100 ms after the R wave. The images were obtained using a flow dephasing gradient strength of 1.2 G/cm. The slice thickness, TE, FOV, matrix dimension, and number of averages were 1 mm, 22 ms, 30 mm, 256 x 256, and 2, respectively. Additional abbreviations are: L.A., left atrium; R.A., right atrium; Ar, aorta; If Ar., leaflets of the aorta.

(-4 @I

Fig. 5. Rat thoracic cardiorespiratory gated 2DFT spin-echo images obtained using a 7 T, 150 mm vertical magnet system. Image (A) was acquired 100 ms after the QRS wave. Imaging parameters for slice thickness, TE, FOV, matrix size and number of averages were 1.5 mm, 20 ms, 48 mm, 256 x 256 and 1 respectively. The in-plane resolution was 190 pm. Image (B) was acquired superior to the heart at the level of the thymus gland with presaturation pulses to reduce blood flow artifacts. Slice thickness, TE, FOV, matrix dimension, and number of averages were 2 mm, 20 ms, 48 mm, 512 x 512, and 2, respectively. In this case, a 100 pm in-plane resolution was achieved. Assignments: Thy., thymus; A. Ar., ascending aorta; D. Ar., descending aorta; S. Vc., superior vena cava; P.A., pulmonary artery; Oes., Oesophagus.


Fig. 6.

Fig. 7.

Figs. 6. and 7. Multi-slice mouse thoracic cardiorespiratory gated 2DFT spin-echo images triggered 25 ms (Fig. 6) and 75 ms (Fig. 7) after the R wave. Slice thickness, TE, FOV, matrix dimension, and number of averages were 1.5 mm, 22 ms, 25 mm, 256 x 256, and 2, respectively. In both figures, images in row A were obtained without flow compensation while those given rows B and C were acquired with flow dephasing gradient strengths of 1.2 and 7.2 G/cm, respectively.

first slice corresponding to end-diastole. As there is minimal turbulent flow in the left ventricle at this end stage of the cardiac cycle, maximum signal from blood is expected. When the heart moves into systole (slices 2 and 3) flow becomes more complex resulting in neg-

ligible signal due to flowing spins not being refocussed during the spin-echo period. A diagrammatic represen- tation of the cardiac cycle with respect to the QRS wave is depicted in Fig. 2A.

This observation was verified by increasing the trig-


gering delay to 75 ms. In this case the heart is imaged predominantly during diastole, but before end-diastole. The results for this experiment are shown in Fig. 7. The images were acquired with the same slice frequencies as those given in Fig. 6. In this experiment, negligible blood flow enhancement signal was detected in any slice. This observation confirms that ventricular blood signal is more pronounced at end-diastole when employing 2DFT spin-echo imaging techniques.

A number of flow suppression sequences have been reported which minimize vascular and ventricular blood flow enhancement signal. The ability to image the heart at end-diastole is important in assessing cardiac status, such as left ventricular mass.’ One approach is the use of spatial presaturation pulses to saturate blood magnetization flowing into the imaging region.i3 This method was applied to image the ascending and descending aorta and pulmonary artery in the rat heart with minimal flow enhancement (Fig. 5A). Alterna- tively, “Black Blood” MR imaging has been proposed where vascular signal is reduced by use of a weighted blood Ti’nulling preparation sequence.15

Our approach to this problem was to utilize a combined spin-echo/gradient refocussed sequence which was initially proposed for imaging of flowing spins.*,‘* The modified sequence for suppression of flow enhancement is given in Fig. 1. With this sequence, both flowing and stationary spins are rotated to the transverse plane by the first nonselective 90” pulse. The symmetrical gradients and the 180” pulse refocus the stationary spins along they axis where they can be re- turned to the longitudinal axis by the last x phased 90” pulse. The advantage in using a spin-echo sequence is

that both chemical shift and B. inhomogeneity are refocussed in the experiment. The phase of the last pulse can be either +x or -x. The flowing spins, however, are dephased by the gradient leaving only a component of the flowing spin magnetization to be refocussed, the rest is spoiled by the first imaging slice gradient. To re- move the possibility of any gradient modulated “stripe” artifact,15*16 the flow dephasing gradients are applied in the slice direction. It is interesting to note that the sequence can be adapted for angiography by replacing the last pulse with a y-phased 90” pulse.*,‘**” Magne- tization from the flowing spins in this case is restored along the z axis leaving the stationary spins to be dephased by the imaging slice gradients.

Experimental results employing two different flow dephasing gradient strengths are given in Figs. 6 and 7. At end-diastole it was found that a dephasing gradient strength of 7.2 G/cm was required to significantly reduce flow enhancement in the left ventricle. This can be visualized when comparing slices 1 and 3 in rows A and C (Fig. 6), respectively. Alternatively for imaging during diastole, a gradient strength of only 1.2 G/cm was required to reduce blood signal, (compare slices 1 and 2 in rows A and B, Fig. 7). The necessity for an increased flow dephasing gradient strength at end- diastole may indicate either slow non-turbulent ventricular flow or the direction of flow not being entirely in the slice direction. Arterial flow that is more laminar in nature is easily suppressed with reduced gradient strengths (Fig. 8).

An alternative sequence that utilizes asymmetrical bipolar gradients ” did not yield better flow enhancement suppression. Both sequences yielded image “stripe”

Fig. 8. Two selected thoracic mouse images obtained from a multi-slice cardiorespiratory gated imaging experiment. The imaging parameters are the same as those given in Figs. 6 and 7. The image on the left represents a control image (without flow compensation) while that given on the right was acquired with a flow dephasing gradient strength of 1.2 G/cm. Note that with flow suppression flowing blood signal is reduced in both the aorta and left ventricle.


artifacts when the flow dephasing gradients were placed in either read or phase directions both in vivo and flow phantoms. Due to the difficulty of being able to suppress flow only in the slice direction, cardiac images at end-diastole were never completely void of blood signal.

Nevertheless, the results presented from this study demonstrate that rodent cardiac images with < 100 pm in-plane resolution and minimal signal from vascular and ventricular blood flow enhancement can be obtained using vertical bore magnet systems employing the abovementioned microimaging methodology.

Acknowledgment-This research was supported by the National Health and Medical Research Council of Australia (F.O.Z.).

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High resolution high field rodent cardiac imaging with flow enhancement suppression

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