FieldStrength – Issue 34 – May 200834

Application tips

Tips for black blood imaging with dual inversion prepulse

Black blood (BB) imaging is routinely used to visualize cardiac morphology. BB imaging can be obtained in two ways. In SE-based BB techniques the outflow effect is used to suppress signal of blood spins flowing through the slice. Alternatively, a dual-inversion prepulse can be applied to null the magnetization of blood in the imaging slice. Below are tips to optimize sequences using a dual-inversion prepulse.

Tip 1: Avoiding myocardial signal loss

Tip 2: Optimizing blood suppression

BB slice 20 mm, imaging slice 8 mm. No myocardial suppression.

BB slice 10 mm, imaging slice 8 mm. Myocardial suppression is seen at the free lateral wall of the LV. Note also the improved suppression of slow flowing blood in the RV (see tip 2).

BB slice 12 mm, imaging slice 8 mm.Good blood suppression.

BB slice 20 mm, imaging slice 8 mm.Incomplete blood suppression.

Wendy de Kok, MR

Applications Specialist

If slow flow or in-plane flow occurs, re-inverted spins within the BB slice thickness may not be completely replaced by nulled blood spins from outside the blood slice at the time of acquisition, resulting in high signal in the blood pool. Reduce BB slice thickness in this case. As a rule of thumb BB slice thickness should be:

2 to 2.5 x imaging slice thickness for through-plane • flow (SA)1 to 1.5 x imaging slice thickness for in-plane flow • (e.g. 4CH)

In Philips preset procedures BB slice thickness is 2.5 x imaging slice thickness.

As the heart is contracting and relaxing during the cardiac cycle, BB slice thickness is usually larger than the imaging slice thickness to avoid myocardial spins moving into the imaging slice and thus showing reduced signal intensity. This effect may occur on SA images in the lateral free wall of the heart, where myocardial motion is largest. Increase BB slice thickness to avoid signal loss.

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Tip 3: Blood suppression for in-plane flow

Tip 4: Easier BB imaging with asymmetric TSE

TE 9 ms, profile order low-high, shot length 102 ms. Halfscan may lead to some residual motion blurring.

TE 9 ms, profile order asymmetric, shot length 130 ms.

TE 60 ms, profile order asymmetric, shot length 130 ms.

Trigger delay mid-diastole, dual-inversion prepulse in same RR-interval.

Trigger delay user defined, dual-inversion prepulse in previous RR-interval.

In case of in-plane flow, as in the aortic arch, spins have to flow over a long distance within the field of view before being refreshed by inverted, nulled blood spins. To benefit from the highest flow velocity of spins, apply the dual-inversion prepulse before systole (end-diastole in the first RR-interval) and acquire the data after systole (early systole in the second RR-interval). Set trigger delay (on the motion page) to user defined instead of mid-diastole to achieve this.

Set the trigger delay to about 0.7 x TR to start the acquisition of the TSE (or TFE) shot at early diastole in the second RR-interval. In the preset procedure TR is set to 2 beats. The actual TR (in ms) and the BB prepulse delay time are displayed on the Info page. Note that acquisition in early-diastole may slightly increase cardiac motion blurring.

TSE is the most commonly used sequence for BB imaging. To avoid cardiac motion blurring, limit data acquisition to the part of the cardiac cycle with least cardiac motion. In general this is mid-diastole.

Adapt TSE shot length for each patient to optimally fit in the patient’s cardiac cycle. Echo spacing must be short to avoid blurring.

Since release 2, asymmetric TSE (a-TSE) enables user defined echo spacing and easy setting of shot length by choosing echo spacing and TSE factor. Echo time TE can be selected freely to optimize contrast in the image.

Visit www.philips.com/NetForum to view extended versions of these application tips or to download a black blood imaging ExamCard for 1.5T or 3.0T.

FieldStrength – Issue 34 – May 200836

Tip 6: Reduce the effect of heart rate variations in T1W_BB_TFE

Tip 7: BB imaging using respiratory navigator

BB imaging requires some form of respiratory motion compensation. Breath holds may not be preferred in multislice sequences, as each slice requires a breath hold. Alternatively a navigator signal can be used. To optimally determine the position of the diaphragm, the navigator signal should clearly differentiate lung and liver, so that resulting images are free of residual respiratory motion effects.

Fat signal from tissues close to or in the navigator beam, may reduce the efficiency of the navigator. It is therefore advised to enable fat suppression (SPIR or SPAIR) if a respiratory navigator is used, particularly at 3.0T.

TE 9 ms, profile order asymmetric, shot length 88 ms. TR 1 beat (923 ms).

TR / TE 5.1 / 3.1 ms, flip angle 25 degrees, shot length 88 ms.

TR / TE 5.1 / 3.1 ms, flip angle 25 degrees, shot length 120 ms.

Tip 5: T1-weighted BB imaging using TFE

No fat suppression. SPAIR fat suppression.

T1-weighted TFE BB sequence, 3 NSA. Some image blurring due to heart rate variations.

T1-weighted TFE BB sequence, 2 NSA and shot interval of two beats. Reduced image blurring.

Due to heart rate variations during a multishot scan, some shots may be acquired in a different part of the cardiac cycle than planned, and cardiac motion blurring may occur. This effect can be reduced by acquiring only one shot per two RR intervals (instead of every RR interval), as the average duration of two RR intervals is usually more stable than the duration of one RR interval.

Modify the TFE shot interval (contrast page) to acquire only one shot per two RR intervals. This will double total scan time, but SNR increases due to longer T1 relaxation. This signal gain can be traded to reduce the number of averages (NSA) or SENSE can be applied to reduce total scan time.

In TSE BB imaging the TR is expressed in beats, so the cardiac cycle determines TR. For PD-weighted and T2-weighted BB imaging, TR is usually set to 2 beats to ensure that TR is long enough.

T1-weighting requires a short TR, but particularly at low heart rates (less than 60-75 bpm), the shortest possible TR of 1 beat is still too long to obtain real T1-weighting. An alternative is a TFE sequence with relatively short TR and sufficiently large flip angle to obtain real T1-weighting. The double-inversion BB prepulse is used to null the blood signal in the imaging slice.

Modify the TFE factor to optimally fit the TFE shot length in the most motionless period of the heart of the patient in diastole.