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Tips for body diffusion weighted imaging (DWI)

Application Tip
Worthington, Paul Philips Healthcare USA

Diffusion weighted imaging (DWI) is typically a single shot echo-planar based sequence that demonstrates pathology based on fluid motion states at the cellular level. Using a single spin-echo RF excitation, k-space is filled by rapid positive/negative oscillation of the readout gradient. Additional diffusion gradient pre-pulses are applied based on the desired b-values to sensitize the image to abnormal fluid motion states created by pathology. Unlike brain imaging where b-value is rarely manipulated (b = 1000 s/mm2), body protocols commonly require adjustments to b-value based on the clinical requirements of the scan.

 Basic DWI pulse sequence diagram
Basic DWI pulse sequence diagram

EPI-based data collection is well suited to high temporal resolution required for imaging of water motion at the cellular level. However, the sequence is also sensitive to magnetic susceptibility phenomena created by air/soft tissue interfaces common in torso imaging. Chemical shift artifacts are also magnified due to the EPI readout and resulting low bandwidth in the phase direction.

The critical technical considerations for diffusion imaging in the body are: 
-Adjustment of b-value to sensitize the sequence to pathology
-Complete fat suppression to eliminate chemical shift and ringing artifacts at fat/water interfaces.
-Reduce susceptibility phenomena by optimizing voxel size, water/fat shift, and parallel imaging (SENSE).
-Motion compensation due to breathing.

    Adjustment of b-value

    The b-value parameter adjusts the strength, duration and time of the diffusion gradients. Unlike brain imaging where b-value is well established (b=1000 s/mm2), b-value requires optimization based on body part, pathology, and radiologist preference. In body imaging, a b-value of 800 is a good starting point in liver imaging for lesion characterization. In other body applications, a b-value other than 800 may be desired based on the pathology. As always, consult the radiologist for the specific b-value to be used.

    A single low b-value (10) can be acquired in a breath hold (< 20 seconds) and is useful in liver imaging to distinguish lesion from blood vessel. DWIBS (Diffusion-weighted Whole body Imaging with Background body signal Suppression) is another example of single b-value DWI used for rapid imaging protocols. For DWIBS, a higher NSA value is used to compensate for breathing motion and SNR loss associated with higher b-values.

    A minimum of two b-values must be acquired if post processed ADC maps or calculated ADC values are desired. Typically, the first b-value is very low (0-100) and the second is tailored to the pathology. ADC maps are generated on the scanner either manually using the Diffusion software or automated as stored with the scan card.

    Multiple b-values can be acquired in a single pulse sequence; however, scan time increases as number of b-values increases. SNR decreases as b-value increases and commonly requires a higher NSA value to regain signal. In this case, motion compensation techniques are used to eliminate breathing motion with longer scan times.

    Fat suppression

    Complete fat suppression is necessary to eliminate artifacts from the increased phase shifts between fat and water. A frequency selective fat suppression pulse (SPIR/SPAIR) with volume shimming is commonly used to eliminate fat signal. SPIR is typically more time efficient and lower SAR relative to SPAIR. However, SPAIR can be a good alternative for larger FOVs to yield more uniform fat suppression with a modest increase in scan time.

    As with any spectral suppression technique, fat suppression non-uniformity is common in areas with higher magnetic susceptibility issues. Air/soft tissue interfaces can be especially problematic in body imaging; consequently, proper placement of a volume shim will be critical to spectral fat suppression. For best results, minimize air within the voxel as shown here:
     <em>Placement of volume shim for optimum fat suppression in liver imaging</em>
    Placement of volume shim for optimum fat suppression in liver imaging

    DWIBS uses an inversion recovery (IR) based form of fat suppression much like conventional inversion recovery sequences. The level of fat suppression is controlled by manipulation of the TI parameter based on the scanner field strength. Inversion recovery based suppression typically results in longer scan times due to the nature of IR-based pulse sequences. However, IR-based fat suppression is less sensitive to B0 inhomogeneity and can be a useful alternative for improving fat suppression if SPIR/SPAIR yields unsatisfactory results.

    Reduce susceptibility effects

    Proper optimization of body EPI sequences requires achieving the lowest possible water/fat shift (WFS). Low WFS implies use of a wide receiver bandwidth, which results in lower SNR but less distortion. Other parameters contribute to reducing water/fat shift as follows:

    SENSE (Sensitivity Encoding)
    SENSE is a parallel imaging technique that can also be used to control susceptibility artifacts common to EPI sequences. Phased array coils must be used and a reference scan performed whenever SENSE is enabled. Susceptibility artifacts are decreased as the P-reduction factor increases, but also result in lower SNR and potential for parallel imaging artifacts.

    P reduction factor = 2 is a good reference point for balancing artifact reduction with SNR loss. Higher P reduction factors will help further minimize WFS with the penalty of additional SNR loss.

    Use a sufficiently large FOV to cover the anatomy without aliasing to minimize SENSE artifacts, or use fold-over suppression when a smaller FOV is desired.

    When SENSE artifacts occur, fold-over suppression and/or a larger phase FOV will eliminate SENSE artifacts.

    Voxel dimensions
    Large voxel sizes are used to minimize susceptibility effects in EPI sequences. If the integrated Q-body coil is used instead of a phased array coil, disable SENSE and use a larger voxel size to help minimize distortion.

    Use a sufficiently large FOV to cover the anatomy without aliasing to minimize SENSE artifacts.

    System performance

    PNS mode = high to facilitate maximum gradient performance and minimize distortions.
    Gradient mode = maximum to facilitate maximum gradient performance and minimize distortions.

    Motion correction

    A variety of motion correction techniques can be used in body DWI to minimize the effects of breathing. Each method will have associated advantages and limitations that make each more or less attractive based on the clinical requirements.

    Breath hold
    This option uses a simple breath hold to control artifacts from breathing. The DWI_BB_BH sequence in the Philips liver folder is optimized for a single breath hold, single b-value scan in approximately 20 seconds.

    Navigator based triggering
    This option uses a 2D navigator for breathing artifact reduction. Navigator uses the right hemi-diaphragm as the physiologic trigger and does not require the respiratory gating sensor. Position the navigator on the dome of the liver using a free-breathing sagittal and coronal scout as shown:
     <em>Placement of navigator on right hemi-diaphragm using sagittal and coronal reference</em>
    Placement of navigator on right hemi-diaphragm using sagittal and coronal reference

    -Navigator respiratory compensation = trigger and track.
    -Gating window = 5 to balance motion compensation with scan time. Higher values will allow more data points/TR to be collected which reduces scan time and potential increased motion artifacts.
    -Alternately, lower values will improve motion artifact reduction at the cost of a longer scan time.
    -NSA = 4 to compensate for SNR loss common with high b-values used in body DWI.
    Respiratory triggering using sensor
    This option uses respiratory triggering and requires use of the respiratory sensor. The trigger delay will need to be optimized based on the respiratory rate to assure data collection falls in the expiratory phase. An NSA value of 4 is used to compensate for SNR loss common with high b-values used in body DWI.
     Placement of respiratory sensor on abdomen over area of greatest abdominal wall motion</em>
    Placement of respiratory sensor on abdomen over area of greatest abdominal wall motion


    Free breathing without gating

    Use this option if respiratory triggering/navigator are not available. Use NSA value = 6-8 for averaging out motion artifacts from breathing.

    Constructing a DWI ExamCard

    The following will illustrate examples of manipulating DWI pulse sequences for use in body imaging based on pulse sequences found in the Philips library.
    The following sequences are stored in the Philips library liver folder:

    DWI_BB_BH: A short breath-hold sequence with a single low b-value (20).
    DWI_3b_RT: A respiratory triggered sequence with three b-values (0,300, 600) and using frequency-based fat suppression (SPIR).
    DWIBS_QBC: DWI sequence using the body coil, single b-value and IR-based fat suppression (STIR).
    DWIBS_STC: Same as QBC sequence but optimized for the SENSE Torso coil.
    DWIBS_TXL: Same as QBC sequence but optimized for the XL-Torso coil.

    DWI_BB_BH

    This sequence has been optimized to obtain a single b-value in a breath hold scan time. SPIR-based fat suppression is used to eliminate artifacts from fat/water shifts. Note the following under the sequence tabs:

    Geometry tab
    Voxel resolution = 2.5 - 3.0mm RL / 2.5 - 3.0mm AP (transverse plane with AP phase direction)
    Slice thickness = 4-8 mm (based on clinical requirements)
    FOV = based on body habitus
    Fold-over suppression if smaller FOV needed.
    SENSE = Yes
    P reduction factor = 2.0

    Contrast Tab
    TR/TE = shortest
    Half-scan = yes
    Water-fat shift = minimum
    Shim = volume
    Fat suppression = SPIR
    Strength = strong
    Diffusion mode = DWI
    Gradient overplus = yes
    PNS mode = high
    Gradient mode = maximum

    Within the contrast tab, a low b-value (20) is selected for differentiation of lesions from blood vessels. Consider the following when changing the b-value in this sequence:

    Scan time increases as b-value increases due to changes in minimum TR/TE. Multiple breath holds may be required with higher b-values.

    SNR decreases as b-value increases. A free breathing sequence with motion correction may be required at higher b-values if the SNR is unsatisfactory.

    Multiple b-factors will require longer scan times and motion correction techniques for optimum image quality. Although this sequence can be modified for multiple b-values and motion correction, the user may find the DWI_3b_RT sequence easier to adjust in this regard.

    DWI_3b_RT

    This sequence is well suited for developing a variety of high b-value DWI sequences. Multiple customized b-values can be acquired with a minimum of effort. SPIR-based fat suppression is used in this sequence as with DWI_BB_BH. Note the following under the sequence tabs:

    Geometry tab
    Voxel resolution = 2.5 - 3.0mm RL / 2.5 - 3.0mm AP (transverse plane with AP phase direction)
    Slice thickness = 4-8 mm (based on clinical requirements)
    FOV = based on body habitus
    Fold-over suppression if smaller FOV needed.
    SENSE = Yes
    P reduction factor = 2.0

    Contrast Tab
    TR = shortest
    Half-scan = yes
    Water-fat shift = minimum
    Shim = volume
    Fat suppression = SPIR
    Strength = strong
    Diffusion mode = DWI
    Gradient overplus = yes
    PNS mode = high
    Gradient mode = maximum


    Examples of b-value adjustment

    Single b-value
    nr of b-factors = 1
    max b-factor value = 800
    average high b = no

    In this example, a single b-value is acquired at b = 800. Any desired b-value can be acquired by changing the max b-factor value. Because a b factor = 0 is not acquired, no ADC trace maps or calculations will be possible.


    Two b-values
    nr of b-factors = 2
    b-factor order = ascending
    max b-factor value = 800
    average high b = yes

    In this example, two b-values are acquired (0, 800). The first b-factor will always be 0 and the second as entered in max b-factor. ADC maps can be post-processed when two or more b-values are obtained. This would be the most common usage scenario for clinical body DWI.

    Alternately, the user can enter the number of b-values to acquire, the maximum b-value, and select ascending b-factor order. In this case, the system will collect b-values evenly spread between 0 and the maximum b-value selected (inclusive). Note that the DWI_3b_RT sequence is stored in this manner using three b-factors and a maximum b-value of 600. The collected b-values in this case would be one each at b = 0, 300 and 600.


    More than two b-values

    nr of b-factors = 6 (depending on user requirements)
    b-factor order = user defined
    click arrow next to b-factors and enter desired b-values 0, 50, 300, 1000, 1500, and 2000
    average high b = yes

    In this example, a total of six b-values are acquired (0, 50, 300, 1000, 1500, and 2000) as entered by the user. A second b-factor of 0 is automatically added as b-factor 7 by default and cannot be changed. This method gives the user complete flexibility to acquire any desired b-values. Scan time increases with each additional b-value added, and SNR decreases with increasing b-value. As such, an increase in NSA may be required to regain SNR at higher b-values.

    DWIBS sequences

    There are three variations of DWIBS in the liver library optimized for use with the Integrated Body coil (QBC, on Achieva 1.5T and 3.0T), SENSE XL Torso coil (TXL, on Achieva 1.5T and 3.0T), and SENSE Torso coil (STC, for Achieva 3.0T). For DWIBS, IR-based fat suppression is used instead of SPIR-based suppression. If IR-based suppression is desired, select the sequence corresponding to the coil used and modify using the recommendations under DWI_3b_RT for b-value and motion compensation.

    Motion correction techniques

    Breath hold
    The parameter "max slices per breath hold" can be adjusted to obtain a shorter scan time. Assuming TR = shortest, changing "max slices per breath hold" will also adjust TR based on the number entered.

    Respiratory triggering using sensor
    The DWI_3b_RT sequence is programmed to use respiratory triggering. The gating window will need adjustment based on the patient's respiratory rate to assure data collection in expiration. NSA = 4 is used to regain SNR lost by higher b-values used in body DWI.

    Navigator respiratory gating
    This option uses navigator based respiratory gating based on motion of the right hemi-diaphragm. When using this option:

    Navigator respiratory = trigger and track
    Gating window = 5
    Scale factor = 1
    Length = 80

    High - NSA respiratory compensation
    For systems without navigator or a respiratory sensor, respiratory compensation can be achieved by setting NSA = 6-8.


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