case studies

Changes in MRI Coil Technology

Introduction
There have been a number of technology changes in MRI since it’s introduction in the 1980’s. These include changes in magnet technology, gradient technology and coil technology. Coil technology is fundamental in the receiving of the RF signal to enable image acquisition. Early MR systems used linear coil technology and in the 1990’s Phased Array Coils were introduced. Phased Array has now been superseded by Matrix Coils. The advantages of Matrix Coil Technology and Parallel Imaging Techniques will now be explored.

Phased Array Coil Technology
Phased array coils involve a number of (usually 4) small surface coils, receiving signal simultaneously and independently from a signal excitation (McRobbie et al., 2003) They are used to improve signal to noise ratio (SNR), using a small coil, whilst scanning a large field of view (FOV) (Westbrook 1998). Each coil has its’ own receiver, limiting the amount of noise received to its’ small FOV. The speed of this technique is determined by how fast spatial encoding is performed and how fast the echo data can be acquired (Heidemann et al., 2003). Faster and stronger gradients have increased acquisition speed by accelerating image encoding times. Adding further receiver channels would allow for parallel imaging techniques to be achieved

Advantages

• Continual reduction in scan times
• Development of fast imaging methods: Echo-planar imaging; Fast Low angle Shot and Turbo Spin Echo
• Improved spatial resolution
• Reduction in motion artefacts
(McRobbie et al., 2003, Westbrook 1998)

Limitations

• Gradient switching steps and hardware factors place significant limits on how quickly the gradients can be switched. This therefore limits the minimal inter echo spacing and minimum repetition times (Heidmann et al., 2003)
• No further improvements in imaging speed can be achieved using gradient performance alone (Heidemann et al., 2003)
• Coils must be repositioned if area of interest is outside the coil range.

Matrix Coils
These are multi element array coils. They consist of several levels in the z Direction. Each level consists of three coil elements in the right – left direction. The signal from each coil cluster can be received in different ways:

Circularly Polarised (CP) Mode – is the equivalent of phased array imaging. It is optimised to obtain maximum SNR at the centre of the region of interest. One RF channel receives information from one coil cluster, made up of three coil elements
Dual Mode – each coil cluster has two RF channels. The dual mode improves SNR and contrast to noise ratio (CNR), particularly in the periphery. This is because there are more coil elements that will receive signal. This mode also allows for parallel imaging techniques for speed, resolution, more slices or coverage and image quality
Triple Mode – each coil cluster has three RF channels. This mode further improves SNR and CNR, particularly in the periphery of the image (Siemens Medical 2003).

(Siemens Medical 2003)

Advantages

• Whole body coverage with local coil image quality
• No need for patient repositioning or coil reconfiguration
• Highest SNR possible
• Parallel imaging possible in all directions
• Faster work throughput

Limitations

• Lack of advanced software. Obtaining more information faster from more independent receiver coils requires faster software
• The number of independent coils is limited by the number of receiver channels available on the MRI system
(Heidemann et al., 2003)

Parallel Imaging Technology
Instead of relying on increased gradient performance for increased imaging speed, Parallel MRI (PMRI) extracts extra spatial information from multiple independent receiver coils, in parallel (Heidemann et al., 2003). Conventional phased array was used to improve SNR over a large FOV. PMRI is applied to significantly decrease scan times. The spatial information content of the coils in the array is exploited to encode and detect multiple MR echoes simultaneously. The same FOV with the same spatial resolution can be obtained with reduced phase encoding steps. The goal of parallel imaging is to provide either a full K-space matrix or unaliased imaging. This is accomplished by using the spatial information obtained in an array of radio frequency coils, in combination with a special reconstruction strategy.

Advantages

• Parallel imaging has acquisition times two to four times faster than traditional imaging systems, which rely on gradients for image encoding. This is in the phase encoding direction (Heidemann et al., 2003)
• Matrix coil technology allows 3D acquisitions to utilise two phase encoding directions simultaneously to achieve parallel imaging factors up to twelve (i.e. three left to right x four head to foot). This potentially leads to speeds of up to twelve times faster than traditional systems (Siemens Medical, 2003)
• Parallel imaging can also be used to improve resolution, whilst maintaining acquisition times
• Parallel Imaging can also be used to maintain scan time while reducing the slice thickness, therefore increasing the number of slices available
• The PMRI approach can accelerate existing imaging sequences
• Useful for breath-hold studies for patients who are unable to hold their breath for an adequate length of time. The scan time can now be reduced for their comfort

Limitations

• Due to lack of Coil Sensitivity and overlapping dedicated PMRI algorithms such as SENSE, SMASH, GRAPPA, and Space RIP are required for image reconstruction (Bankson and Wright, 2002)
• Pre scan calibration is required to calibrate the coil sensitivities, this is necessary for PMRIi reconstruction. This usually takes approximately one or two minutes, this can increase the patient scan time
• Pre scan calibration can be problematic when the coil sensitivity across the area being scanned has changed between the pre scan and the actual image acquisition i.e. the patient has moved (Bankson and Wright, 2002)

Conclusion
Matrix Coils and Parallel Imaging Techniques can be considered the future for Clinical imaging. The most evident advantage of using PMRI is the reduction in acquisition times. This is fundamental in the use of Matrix Coils as more independent receiver coils means the possibility of achieving faster acquisition times and improved image quality. This is particularly useful for breath-hold studies and uncooperative patients.

References
1. Bankson J. A and Wright S.M. (2002) Simulation-Based Investigation of partially Parallel Imaging a linear Array at High Accelerations. Mag Res Med. April 47 4 p777-86
2. Heidemann R. M, et al., (2003) A Brief Review of Parallel Magnetic Resonance Imaging Eur Radiol 13 p2323-2337
3. McRobbie D. W, et al., (2003) MRI from Picture to Proton. (First Edition) Cambridge: Cambridge University Press
4. Siemens Medical Solutions UK (2003)
5. Westbrook C. (1998) MRI in Practice (Second Edition) Oxford: Blackwell Science Ltd

Lesley Kemble – Senior Mobile Technologist - North Region

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