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2.4.1. RUFIS sequence applied to cardiac imaging

The RUFIS sequence was explored for cardiac imaging as well. For cardiac applications some T1

or T2 contrast needs to be produced in the images, otherwise the muscle will not be

distinguishable from fat or blood. The T1 preparation, the T2 preparation and the fat suppression

modules used for the head imaging were tested on the chest. Moreover a double triggering was implemented in the sequence. A first triggering was on the respiration signal and then a second one on the electrocardiogram. The double triggering had the purpose to synchronize the acquisition with the end-expiration respiratory phase and with the diastole phase of the cardiac cycle.

The obtained results showed that the preparation modules still need further improvement for the cardiac imaging in order to generate informative 3D cardiac images. The fat suppression module showed an image with some contrast on the heart (Fig. 55), but the image quality is not acceptable, comparing to the common 3D sequences used for cardiac imaging.

Fig. 55 RUFIS image of the chest using a fat suppression preparation pulse. Some contrast is visible on the heart, but the image quality is not comparable to the common product sequences used for 3D imaging of the chest (e.g. the LAVA sequence [2]).

2.4.2. Prospective triggering applied to cardiac T2 mapping

In the section about cardiac imaging a solution was shown to better fit the acquisition in the breath hold time. This could be useful for patients which can barely hold their breath for long. Alternatively the acquisition can be extended beyond the BH time using a prospective triggering as described above for lung imaging. This approach could be necessary in the cases of patients which can hardly hold their breath.

2.4.2.1. Implementation

A prospective triggering on the respiration signal was implemented in the T2-mapping MEFSE

sequence in order to extend the acquisition beyond the BH time. For comparison, free breathing (FB) examinations were acquired as well. The FB acquisition was performed with 1, 2 and 3 averages (Navg) in order to reduce the motion artifacts in the images.

The prospective triggering approach was implemented similar to how described above for the RUFIS sequence. The triggering was double, first on the respiration signal, and then on the electrocardiogram within the acquisition window. Short-axis cardiac images were acquired on 6 healthy volunteers at 3T (GE Healthcare, Waukesha, WI, USA). A six-segment model of the heart was considered to measure the average T2 over the myocardium. The analysis of variance

(ANOVA) was applied to estimate the presence of any significant difference among methods and among the segments of the heart for each method. A difference with p-value<0.01 was considered significant.

2.4.2.2. Results and discussion

Eight T2-weighted images were acquired at 11ms TE increments. The scan parameters were:

ETL =16, BWRx=±62.5 kHz, FOV=32, 192x128 matrix. The BH acquisition was performed on

average in 24s scan time with Navg=0.5. For both FB and PT multiple averages were tested. Fig.

necessary to reduce the motion artifacts, although the images appeared blurry. The PT approach avoided most of the motion artifacts already for Navg=1, improving artifact rejection for higher

Navg. However the scan time associated with PT is much longer than FB (Fig. 56). According to

the ANOVA-Bonferroni test, the BH approach did not show any significant difference with the PT approach for any value of Navg. Conversely, the BH approach resulted to be significantly

different from the FB approach for any value of Navg (Tab. 7). Fig. 57 shows the six-segment

model of the heart with associated average T2 values. The ANOVA test resulted in segments

significantly different in all cases. For both the BH and the PT approaches, the segments number 2 and 3 (i.e. the ones on the septum) showed a mean T2 of 53.7ms, whereas the mean T2 in the

rest of the myocardium was 47.7ms. For the FB approach the presence of ghosting and blurring caused an increase of T2 in segment 2 and 3 extended to segments 1 and 4 as well.

Fig. 56 T2 maps obtained in breath-hold (BH), in free-breathing (FB) and with prospective triggering (PT). On top of each image: the acquisition method, the number of averages (Navg) and the average scan time. The same color map was used for all the images, thresholding the T2 values between 30ms and 80ms.

Tab. 7 Results of the ANOVA-Bonferroni test. The BH exam is statistical equivalent to the RT, whereas it is significantly different from the FB.

Fig. 57 T2 maps represented with the 6-segments model of the hear for the BH, the FB Navg=3 and the RT with Navg=1. The septum shows higher T2 values than the rest of the myocardium for BH and RT. For FB the motion artifacts and the blurring generates inaccuracy in the maps. The MEFSE sequence was used to produce T2-maps comparing the breath-hold approach with

the respiratory triggering and the free breathing. The RT was shown to successfully produce T2-

maps statistically equivalent to the maps obtained with the BH approach. The acquisition in the PT approach was not limited to the breath-hold time, making the examination easier to prescribe and more comfortable for the patient. The FB approach showed non negligible motion artifacts which can be reduced only increasing the Navg. However, the use of high Navg with the FB

approach produces blurry maps significantly different from the BH ones. For all methods a significant difference was found among the T2 values of the segments. For both the BH and the

RT approach, the T2 values of the septum appeared 12.6% higher than in the rest of the

myocardium. This is due to the fact that the septum is the area of the myocardium mostly protected from artifacts. For the FB approach the T2 values of the segments were mixed because

of the ghosting and the blurring, showing a higher T2 value for the segments immediately