Friday, September 25, 2009

Pulse Sequences for Cardiac MRI

Bright Blood Technique


Dark Blood Technique

A non-selective 180º inversion pulse excites all the tissues and blood within the entire heart when the R-wave trigger is detected at the beginning of the cardiac cycle. Immediately following, a slice-selective re-inversion pulse excites only the tissues and blood within the image slice. The net result is that everything within the slice is flipped back to normal because it experienced both the inversion and re-inversion pulses, whereas everything outside the slice remains inverted. Next, the systolic contraction forces the blood within the slice to be replaced by blood from outside the slice. After enough time delay has occurred to allow the blood to be fully replaced within the slice, the image data is collected during mid to late diastole of the cardiac cycle.


Double Inversion Recovery

The red line represents the signal of the blood that flows into the image slice, whereas the green line represents the signal of the myocardium within the image slice. Note that the myocardium signal is initially inverted and then immediately re-inverted, so it remains positive during the cardiac cycle. But the blood signal is initially inverted and then gradually recovers toward positive during the cardiac cycle, passing through zero at some time during diastole. The time delay required for the blood signal to pass through zero is known as the blood null time. If the data acquisition is carefully timed to occur exactly when the blood signal is passing through the null point, the blood appears dark in the image.


Triple Inversion Recovery

The third IR pulse and the fat signal are shown in yellow. Although the delay time for blood to cross its null point is relatively long, the delay time for fat to cross its null point is much shorter. If the sequence timing is carefully controlled such that both the blood and fat cross their null points at the same time that data is collected, both blood and fat will be dark in the image.


Short RR Interval

Reduce the data collection segment by reducing the number of lines per segment, or reduce the echo spacing by increasing the bandwidth and/or using faster RF pulses and gradient pulses. If the pulse sequence is single shot - Haste or TrueFisp, reduce the data collection segment by reducing the Phase FOV or Phase matrix, use the minimum possible TE & iPAT.


Long RR Interval


TI Scouting for Delayed Enhancement

An IR pulse and cine data acquistion are repeated every second heart beat. Since T1 recovery is occuring during this period, as shown by the yellow curve, each cardiac phase has a different TI relative to the initial IR pulse.


Inversion Recovery - TurboFLASH for Delayed Enhancement

T1-weighting is maximized by using a 180 degree inversion pulse to flip the signal of all tissues to the negative axis. The signals of tissues with different T1 recovery rates are allowed to separate out over time, as shown by the red and yellow curves. The total recovery period spans through 2 cardiac cycles to allow enough time for all tissues to fully recover back to their positive axis. During this recovery period, the necrotic tissue has higher signal than viable tissue due to its faster T1 recovery rate. Data is collected ideally when there is maximal separation between necrotic and viable tissues. Maximal contrast between necrotic and viable tissues is achieved if the TI is adjusted so that the viable tissue is crossing the null point when the data is collected.


Phase Sensitive Inversion Recovery for Delayed Enhancement






Saturation Recovery - TurboFLASH for Myocardial Perfusion

2 comments:

  1. Anonymous8:58:00 AM

    Excellent explanations - Thank You

    ReplyDelete
  2. Anonymous10:29:00 AM

    very usefull, thank you so much from Chile

    ReplyDelete