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Strategies for faster scanning in orthopedics

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

When looking for ways to make your musculoskeletal MRI scans faster, you may benefit from several strategies described here to speed up orthopedic MR imaging.

A range of techniques is discussed, including Compressed SENSE, dS SENSE, mDIXON TSE and parameter optimization.

Tip 1: Use Compressed SENSE to scan faster with virtually equal image quality

Compressed SENSE enables MRI scan times that are up to 50% faster than without Compressed SENSE, with virtually the same image quality. It is typically less sensitive to noise breakthrough than other acceleration techniques.

Compressed SENSE can be used across your ExamCard, in all anatomical contrasts and for both 2D and 3D sequences.
Hip without (top row) and with Compressed SENSE (bottom row), saving 40% scan time Performed on Ingenia 1.5T. Courtesy of Kurashiki Central Hospital, Japan
Hip without (top row) and with Compressed SENSE (bottom row), saving 40% scan time
Performed on Ingenia 1.5T. Courtesy of Kurashiki Central Hospital, Japan

To see more about Kurashiki Central Hospital's experience with accelerating exams using Compressed SENSE, please visit the
 FieldStrength article: High quality and fast scanning with Compressed SENSE at KCH


Tip 2: dS SENSE parallel imaging for MRI systems with dStream

When Compressed SENSE is not available, the dS SENSE parallel imaging technique is often used to reduce scan time. It is available for fully digital Philips MRI systems.
 
It reduces scan time by reducing the number of phase-ecoding steps, based on the coil sensitivity data. Two or more coil elements are used to acquire imaging data simultaneously, enabling a scan time reduction by a factor of up to the number of coil elements used.

The dS SENSE factors in the Philips database are set to balance acceleration, SNR and resolution.  Use these acceleration factors as a guide, and adjust to your clinical need.

Care should be taken when using dS SENSE to ensure that anatomy is covered either by the imaging FOV or by oversampling.
 

Tip 3: mDIXON TSE for obtaining multiple contrast types in one acquisition

mDIXON can speed up your examination by delivering multiple image contrast types within the same acquisition.

The mDIXON method acquires in- and out-of-phase images (IP, OP) and uses this data to reconstruct water only, fat suppressed (WO) and fat only (FO) images. 

For example, when an ExamCard includes sagittal PD TSE and sagittal PD TSE fat suppressed scans, these two sequences can be replaced with one single sagittal PD mDIXON TSE scan.
mDIXON TSE delivers IP and WO images in the same scan time as standard TSE Performed on Ingenia Ambition 1.5T. Courtesy of Spital Uster, Switzerland.
<br>
mDIXON TSE delivers IP and WO images in the same scan time as standard TSE
Performed on Ingenia Ambition 1.5T. Courtesy of Spital Uster, Switzerland.
 
Related information on NetForum:
 

Tip 4: Parameter optimization for faster scans

Advanced TSE (pseudo steady state)

Advanced TSE can be used to reduce scan times and to reduce the Specific Absorption Rate (SAR) of the sequence. Advanced TSE allows you to control the flip angle sweep across TSE acquisitions using the pseudo steady state (PSS).

In addition to scan time and SAR reduction, Advanced TSE can be used to prolong the shot length, by maintaining the image sharpness while reducing scan time.  Alternatively, Advanced TSE can be used to reduce flow artifact compared to refocusing angle constant.
 

When you set up a sequence with PSS, consider the following:

For T2-weighted images, make use of 'T2-optimized':

    • With lowering the refocusing angles, flow artifacts will become less apparent
    • A higher 'mid. angle' can help in reducing image blurring
    • A higher 'max. angle' can help in increasing SNR

 

 

For T1-weighted images, make use of 'T1-optimized':

    • 'T1-optimized' is suitable for 2D, MS and M2D.
    • To reduce artifacts, such as truncation artifacts, use 1 startup echo.
    • A large difference between (~40°) the 'min. angle' and 'max. angle' may result in image blurring due to the increased signal modulation close to k0. This is TSE factor and profile order dependent.
    • When you reduce the 'min. angle', then increase the 'max. angle' by the same value.
    • A lower 'min. angle' can help to increase the T1 contrast and to also reduce motion related artifacts.
    • A higher 'max. angle' can help in increasing SNR, however SAR will also increase.

 

When you adjust parameters, always keep an eye on the TEeff / TEequiv

Fold-over suppression

Asymmetric flexible oversampling allows you to achieve fast scan times, by using fold-over suppression only where it is needed.  Where anatomy lies outside the imaging FOV, this should be covered with foldover suppression, to avoid aliaising artifact. 
Flexible oversampling: Shoulder An example of flexible oversampling, used to 
cover more thorax (Foot direction) than neck (Head direction)<br>F: 80 mm<br>Flexible oversampling: Knee An example of flexible oversampling, used to 
cover the contralateral knee (Left)
Flexible oversampling: Shoulder
Flexible oversampling: Knee
An example of flexible oversampling, used to cover more thorax (Foot direction) than neck (Head direction)
F: 80 mm
An example of flexible oversampling, used to cover the contralateral knee (Left)

Number of Samples 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%.

 

From Release 5, partial NSA can be used to gain extra signal, without the need to scan an entire integer NSA.  For best results, use partial NSA more than x.5 (eg NSA 1.8, 2.6) and avoid using partial NSA less than x.5 (1.3, 2.4)

Select the most appropriate coil

Selecting a coil that yields the SNR allows for scan speed reductions.

Dedicated coils such as the dS Knee, dS Shoulder, dS Small Extremity coils provide greater SNR than general coils, such as dS Flex coils. The selected coil should match the required anatomy, FOV and matrix.

Note that the Philips database protocols reflect optimized parameter settings, to achieve a balanced trade-off between benefits and drawbacks.

Check the overall effect of your parameter changes

When customizing the scan, check Dashboard and Info page for the effect of your paramter changes. Parameter changes are indicated in bold font, and the original value is indicated in brackets. Check all relevant parameters, and particularly

  • scan time
  • voxel size
  • relative SNR
  • scan percentage
  • TSE echo-spacing / shot length

 

 Check for the overall effect of your parameter changes: the Dashboard (highlighted green) and Info page (highlighted blue) indicate the effect of parameter changes
Check for the overall effect of your parameter changes: the Dashboard (highlighted green) and Info page (highlighted blue) indicate the effect of parameter changes


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Application Tip
Compressed SENSE, dS SENSE, dStream, mDIXON, mDIXON TSE, Musculoskeletal
 

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