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T2* (T2 star) calculation of the myocardium

Application Tip
Springorum, Rudolf Philips Healthcare

Introduction

Recent publications have proposed several ways to calculate the T2* of the myocardium. The basic idea is to image the myocardium a few times with different TE's, using a gradient echo technique. The change in signal intensity across the echoes follows a T2* decay.

 

The first publications use separate scans for each TE. This means that the values from different acquisitions are taken to do the T2* calculations. The automated signal scaling requires that those individual scans need to be calibrated. Often the standard deviation in the background noise is used to calibrate the individual scans. But with the introduction of SENSE and CLEAR this background noise level varies across the field of view. Other publications [1] explain the use of true multi-echo scans.

 

There is however another interesting approach proposed by for example Li et al [2]. They measure just two echoes for a T2* calculation and draw ROI's to calculate the T2*, and that is exactly what we can do on our system nowadays.

Acquisition

A black blood dual echo cardiac triggered TFE scan was developed to meet the requirements for T2* calculation. The resolution is kept low just to aim for the best possible signal to noise. Because the T2* values are derived from two sample points, the signal-to-noise ratio needs to be good. For the same reason the water-fat shift is set at maximum. Both TE's are chosen such that water and fat are precisely in phase, just to avoid any influence from intra voxel water and fat de-phasing. At 1.5T the TE's are set to 4.6 ms and 9.2 ms. At longer TE's the signal of flowing blood gets disturbed and flow artifacts start to appear. For this reason a dual inversion black blood pulse - to cancel the signal of blood completely - is added to the sequence. The scan is cardiac triggered and the TFE shot is kept reasonably short to avoid any cardiac motion. The whole scan is performed in a single breath-hold or free breathing using a navigator. Both protocols are available in an ExamCard.

Processing

To calculate the T2* a contour is drawn over the septum on one image and copied to the other echo. The mean value of both ROI's are taken to do the calculation.

First echo at 4.6 ms Second echo at 9.2 ms ROI statistics
First echo at 4.6 ms
Second echo at 9.2 ms
ROI statistics

The T2* decay can be expressed using the following equation [A]. This equation can be rewritten such that we can calculate T2* if the signal intensities at both TE's are known [B].

 

In our example  the T2* is calculated as follows:

T2* = -(9.2-4.6) / ln(1216/1424) = -4.6 / ln(0.85) = -4.6 / -0.158 = 29 ms

[A] T2* decay The Signal intensity (I) at time (t) equals the intensity at time zero multiplied with the exponential component that describes the decay (e exp -t/T2*).[B] Calculate T2* The T2* value (in ms) is the time between the two echoes (delta TE's) divided by the natural logarithm of the division of signal intensity at TE2 by the intensity at TE1.
[A] T2* decay
[B] Calculate T2*
The Signal intensity (I) at time (t) equals the intensity at time zero multiplied with the exponential component that describes the decay (e exp -t/T2*).
The T2* value (in ms) is the time between the two echoes (delta TE's) divided by the natural logarithm of the division of signal intensity at TE2 by the intensity at TE1.

Results

The scan method as presented here was validated in-house on healthy volunteers. The mean T2* value found was 32 ms (+/- 4 ms, n=11) which falls well within the range as described in literature. Our findings correspond extremely well with the findings of Li [2] (mean = 33, +/- 5.6, n=13) who uses a similar dual echo technique.

Note

Although the method presented minimizes the influences of motion, water and fat out-of-phase effects and inflow effects, the accuracy of T2* measurements can be limited by other effects that influence the signal intensities of both echoes.

References

[1] Anderson LJ, Holden S, Davis B, et al. Cardiac T2-star (T2*) magnetic resonance for the early diagnosis of myocardial iron overload. European Heart Journal 2001; 22, 2171-2179.

 

[2] Li D, Phawale P, Rubin PJ, et al. Myocardial signal response to dipyridamole and dobutamine: Demonstrating the BOLD effect using a double echo gradient echo sequence. Magn Reson Med 1996; 36: 16-20



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May 29, 2005

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Application Tip
Achieva 1.5T, Intera 1.5T
Release 1, Release 11
Cardiac, Cardiac Morphology, Function Cine, Pediatric
 

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