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Strategies for faster scanning in orthopedics

Application Tip
Bloemers, Koert Philips Healthcare Philips Global
Gulpers, Sjef Philips Healthcare Philips Global

 

This article discusses strategies to speed up orthopedic imaging by decreasing the number of k-space lines actually measured, in order to reduce scan time. As you're always balancing scan time and image quality, deciding to add one or more of these methods to your scan, depends on the method's side effects and on your personal preference and understanding of the technique and its effect.

Check the "Info Page" repeatedly

When optimizing a scan, check the info page frequently to compare scan time, RSL (relative SNR level) and Voxel Size.

Select the right Coil / FOV / Matrix - combination

Selecting the coil that yields the best SNR enables greater scan speed reductions.

The SENSE Flex-M coil (shoulder, knee), SENSE Flex-S (elbow, wrist), SENSE Body coil (hips) and Quadrature Knee coil provide better SNR than C- (C1, C3, C4) and E- (E1) surface coils. The selected coil should match the FOV and matrix needed.

Optimizing for faster orthopedic scanning

Note that the Philips factory protocols reflect our recommendations to achieve the best trade-off between benefits and drawbacks. Short descriptions of the methods and application tips are given here. Consult the Application Guide for more information.

 

OPTIMIZE PARAMETER SETTINGS

Rectangular field of view (RFOV)

The distance between k-space lines is increased, so that the image size is reduced in the fold-over direction, resulting in a rectangular FOV. Scan time is proportional to the percentage that indicates the ratio between the short and long sides of the FOV. Using RFOV does not affect spatial resolution - voxels are still square, but SNR is reduced.

 

  • Reduce the rectangular FOV percentage so that the anatomy just fits in the FOV, with fold-over direction AP or RL for knee, ankle, wrist or elbow. We recommend maintaining the RFOV at greater than 50%. Check during planning that there is no anatomy in the fold-over direction that could fold back into the image. Avoid RFOV for shoulder or hip imaging, because backfolding is more likely to occur there.

 

Scan percentage

The outermost profiles of k-space are not scanned, but filled with zeroes. This reduces scan time (= proportional to scan%) and improves SNR (because a line of zeroes contains less noise than a measured line), but does so at the cost of fine spatial resolution, which is encoded in the high k-space profiles.

 

  • Reduce the scan percentage when spatial resolution is not critical. Keep the percentage above 60% for best results. This method is simple and robust and may well be combined with RFOV.

 

Knee imaging Standard T1wTSE, FOV 160 mm, acq. matrix 256, 3 NSA, scan percentage 80%, scan time 2:37 min.Knee imaging Scan percentage 65%, scan time 2:08 min.Knee imaging Rectangular FOV 80%, scan time 2:07 min.
Knee imaging
Knee imaging
Knee imaging
Standard T1wTSE, FOV 160 mm, acq. matrix 256, 3 NSA, scan percentage 80%, scan time 2:37 min.
Scan percentage 65%, scan time 2:08 min.
Rectangular FOV 80%, scan time 2:07 min.

Flexible matrix

Scan time is proportional to matrix size. On Intera and Achieva systems, the matrix size can be set freely: either to any value or to a multiple of 16 - between 64 and 1024. This flexible matrix is a powerful tool for optimizing scans: it allows scan time reductions in small steps, while keeping voxel size constant and adapting FOV to coil or anatomy size.

 

Full matrix versus flexible matrix wrt aptip 'Strategies for faster scanning in orthopedics'
Full matrix versus flexible matrix
wrt aptip 'Strategies for faster scanning in orthopedics'

Example (see figure above):

With FOV 200 mm, matrix 256, the FOV is larger than the SENSE Flex-S coil. So reduce the scan time and gain SNR by reducing FOV to 150 mm and matrix 192.

 

  • Use flexible matrix: reduce matrix size and FOV to reduce scan time without changing voxel size - check this on the Info page. Watch the SNR (RSL) during planning.

 

Full matrix FOV 160 mm, acq. matrix 256, voxel 0.6 mm, scan time 2:37 min.Flexible matrix FOV 140 mm, acq. matrix 224, voxel 0.6 mm, scan time 2:18 min.
Full matrix
Flexible matrix
FOV 160 mm, acq. matrix 256, voxel 0.6 mm, scan time 2:37 min.
FOV 140 mm, acq. matrix 224, voxel 0.6 mm, scan time 2:18 min.

SENSE

With SENSE, two or more coil elements acquire imaging data simultaneously, enabling a scan time reduction by a factor of up to the number of coil elements used.

 

  • When applying SENSE, position the SENSE Flex-M or - S coil AP with SENSE direction AP when scanning sagittal slices. Avoid applying coil elements too close to each other, to prevent signal drop due to coupling. Keep SENSE factors below 1.5 for best results in sagittal and coronal knee imaging. Make axial knee images with SENSE factor up to 2.0, 2 NSA and fold-over suppression.

 

Halfscan

Only slightly more than half the k-space profiles are measured, reducing scan time by almost 50%. The missing data is copied from the measured half of k-space. Since noise is also copied, SNR decreases too.

 

  • Use halfscan for scans with NSA=1, when SNR is not critical. In TSE, a halfscan factor can be selected. Use profile order with halfscan to optimize TE.

 

Number of Signals Averaged (NSA)

NSA is the number of times each k-space profile is measured. Scan time is proportional to NSA, so, for instance, halving NSA will halve scan time, but also reduce SNR to 71%.

 

  • Set NSA no higher than necessary. Be aware that the selected NSA determines the fold-over suppression method used by the system.

 

SENSE Knee imaging Scan time 1:49 min.Knee imaging with Halfscan Scan time 2:03 min with Halfscan factor 0.6Knee imaging and NSA 2 NSA, Scan time 1:47 min.
SENSE Knee imaging
Knee imaging with Halfscan
Knee imaging and NSA
Scan time 1:49 min.
Scan time 2:03 min with Halfscan factor 0.6
2 NSA, Scan time 1:47 min.
k-space drawings scperc, rfov, hs with s
k-space drawings scperc, rfov, hs with s


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Application Tip
Achieva 1.5T, Intera 1.5T
Release 1, Release 10, Release 11, Release 9
Explorer / Nova Dual, Master / Nova, Nova, Nova Dual, Omni / Stellar, Power / Pulsar, Pulsar
Ankle, Elbow, Foot, Hand, Hip, Knee, Musculoskeletal, Shoulder, Wrist
 

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