Magnetic resonance imaging (MRI) is usually restricted by the resulting signal-to-noise ratio (SNR). Use of higher field strength and improvement in the radiofrequency (RF) receive coil design are believed to facilitate significantly higher SNR.
In the final MR image, noise is primarily dominated by two factors: RF emission on account of thermally driven Brownian motion of electrons within the body's conducting tissue, and electronic noise introduced by the entire receive chain of the MR scanner, like electronics, receive coil, etc.
At mid and high fields, the patient is often the dominant noise source unless the coil is extremely small. However, in small animal MRI, the imaged volumes being determined are of a size that makes the sample noise contribution equivalent or smaller than the thermal noise contributions of the receiver system, even at increased magnetic fields.
Under these conditions, thermal noise reduction facilitates higher SNR. Earlier, the RF probes were cooled to reduce the thermal noise of the NMR system. This technique was generally applied in small sample NMR experiments.
Cryogenically Cooled Resonators
Cryogenically cooled resonators (CCR) have recently been introduced for small animal imaging applications. This has resulted in significant improvements in SNR by a factor of 4 when compared to the traditional room-temperature probes at 200 or 400 MHz. Even factors in the order of 5 were reported in rapid cardiac imaging application.
The significant gain in SNR may allow further application of MRI in fields, which are presently SNR and/acquisition time limited as e.g. high-resolution diffusion tensor imaging (DTI).
According to hypothesis, the possible gain in SNR for CCRs rises inversely with the field strength, but for small resonators like those applied for mouse brain imaging, considerable gains in SNR are expected even at 500 MHz, allowing higher spatial resolutions or faster imaging.
This technical brief explores the possible gains in spatial resolution or acquisition time at 500 MHz by applying a 2-element CCR in functional and brain cardiac imaging.
In each study, three wild-type mice (C57/B6) were enrolled. The measurements were performed under isoflurane anesthesia (3% for induction and -1.5% for maintenance). Using an MR compatible small animal monitoring system, physiology was monitored.
The breathing frequency was sustained in the range of 50-65 respiratory cycles per minute. A warm water heating system was used to control body temperature. Animals were laid in prone position in both coils throughout the brain study, as shown in figure 1.
For cardiac examinations, the mouse was placed in prone position in the room temperature coil, as illustrated in figure 2, and in supine direction in the cryogenic probe.
Data acquisition was carried on a small animal imaging system, BioSpec 117/16, Bruker BioSpin. All acquisition protocols were conducted two times, each with data reception by a dedicated 4-element brain/heart receive array coil and a dedicated 2-element CCR brain transmit/receive array coil. For the receive-only coils, a quadrature volume resonator (T072, Bruker Biospin) was utilized for excitation.
Brain MRI Protocol
First, para-coronal slices were obtained. In order to guarantee reproducible acquisition geometry, the image plane was aligned with the cortex and the mid-sagittal line, as illustrated in figure 3.
Rotation was not performed around the RL axis to reduce B1-induced intensity modulation over the image. For CCR acquisitions, the reference gain was improved in the central slice of the imaging stack, the degree of which was kept less than 3 mm to prevent B1 - variation over its extension.
All SNR comparisons were carried out with mildlyT2-weighted RARE imaging sequences. Sequences consisted of a high-resolution scan (BHR), a low resolution scan with reduced slice thickness (BMR), and a low resolution (BLR) scan with different number of excitation (NEX).
Cardiac MRI Protocol
The performance evaluation for cardiac applications was based on 2D and 3D cine acquisitions. In the case of 2D acquisitions (CINE2D), cine image were obtained n 2-chamber, 4-chamber, and short-axis geometry, as shown in figure 4.
For 3D cine imaging (CINE3D), the 3D-volume was considered in para-coronal geometry, as illustrated in figure 5. The volume was aligned the heart’s long axis. The great vessels were carefully excluded from the excitation volume to reduce blood saturation, thus ensuring adequate contrast of blood and myocardium. Utilizing a self-gated reconstruction technique (IntraGate, Bruker Biospin), all data was reconstructed.
SNRs were calculated according to:
where |R0| was the mean value of a manually drawn region of interest (ROI), the standard deviation of the background signal, and the mean background signal.
For the brain data, the average SNR of four segments of the right and left hemisphere was utilized for SNR comparisons.
For the cardiac data, the mean SNR was measured separately for the septal, anterior, posterior and lateral wall.
Since in 3D parallel imaging was employed, SNR comparisons were limited to the 2D acquisitions. All ROIs were selected at similar locations to guarantee a fair comparison of the results.
The images acquired were directly compared with the different investigated coils and imaging protocols. This revealed a considerable increase of SNR with the Cryocoil (bottom row) as illustrated in figure 6.
To this end, the respective SNR analysis clearly demonstrates more than a threefold gain in SNR for all explored protocols, as shown in figure 7.
Moreover, all enhancements were highly significant (p-value < 0.01). Using the SNR gain, the CryoCoil allowed high-resolution brain imaging in acceptable acquisition times.
In restructured SNR maps, a substantial increase in the SNR can be appreciated in all regions of the myocardium. Interestingly, the posterior region of the myocardium had the strongest gain, denoting a more homogeneous sensitivity profile of the 2-element coil, which can also be appreciated in the 2-chamber view.
Additionally, reconstruction of the 3D CINE data produced adequate image quality and contrast for the assessment of the contractile motion.
Produced from articles authored by Ina Vernikouskaya, Axel Bornstedt, Volker Rasche.
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