Reliable data related to the iron level in the myocardium can be obtained using GRE sequences and measuring the T2* value. Thus, this approach has become an important diagnostic tool for planning and monitoring the chelation therapy in patients with transfusion-dependent anaemia . To this aim, breath-holding GRE sequences are commonly used . However, these sequences are quite sensitive to respiratory artefacts and always require adequate breath-holding during image acquisition . Thus, this standard technique is not feasible in patients who are unable to perform breath-holding commands, especially in young children.
In our study, we compared the results of the T2* mapping using standard breath-holding technique and a multiple signal accumulation free-breathing technique and found that 14% of the breath-holding studies had a non-diagnostic quality due to the presence of respiratory artefacts. On the contrary, in these patients, free-breathing sequence was of adequate quality and could be used for T2* mapping.
ECG-gated multiple signal accumulation techniques allow to compensate both respiratory activity of a patient. MR scanners with different magnetic field strengths are differently sensitive to the physiological movement, and multiple signal accumulation techniques with ECG-gating have been easily adapted to 1.5-T systems . In our study, we demonstrated the application of ECG-gated multiple signal accumulation technique at 3-T. T2* values were measured in the interventricular septum for patients with good breath-holding, and a strong positive correlation was established between the T2* values obtained using the FB-mTFE and BH-mTFE sequences. Therefore, free-breathing scanning can be used for myocardial iron detection in paediatric practice at 3-T.
In general, the free-breathing multiple signal accumulation scanning technique may provide a higher SNR than a single breath-holding scan. The mean T2* values measured for the same ROI were not significantly different between the two techniques, however the SD of T2* was smaller for the free-breathing sequence than for the breath-holding sequence, corresponding to more homogenous T2* values for the free-breathing myocardial mapping. Therefore, the free-breathing myocardial mapping technique demonstrates higher image quality compared to the standard technique for patients who could not clearly follow the breath-holding instructions, especially in patients aged from 2 to 7 years. Of note, based on our experience, it is rare that patients under 7 years of age are able to methodically hold their breath for 8–12 s.
However, most of the haematologic patients from 5 to 7 years of age can lie in the MR scanner without any movement for 20–30 min. Despite the motionless of this patient group, the usage of the multi-phase GRE sequence without anaesthetic support is impossible due to the huge number of breathing artefacts, which make it difficult to generate the relaxometry maps and calculate the T2* values.
Three of the patients in the age from 5 to 7 years were scanned without anaesthetic support. For these patients, five or more re-scans (repetition of the scans) were conducted for the breath-holding T2* analysis without successful results: images were corrupted by the respiratory artefacts. The duration of the studies was extended from 15 s to 2 min including attempts and preparation time. The use of the free-breathing sequence allows to reduce the number of re-scans due to the fuzzy holding of the patient’s breath and the absence of the breath-holding preparation increases the comfort of the study.
The free-breathing protocol will help to optimise the investigation of patients aged from 2 to 7 years. Some cases will allow to omit anaesthesia or at least to reduce its depth, which is extremely important in paediatric practice. Despite the fact that the duration of the investigation by free-breathing technique with artefact compensation is longer than the standard breath-holding technique, in paediatric practice, the whole duration of the study will be reduced. In most cases, children do not manage to adequately hold breath from the first try, and the study continues with the repetition of the sequences several times. This makes it possible to consider the free-breathing T2* mapping technique potentially promising for the myocardium iron assessment also in other patients who are not able to hold their breath due to their health condition.
Our study has some limitations. This work was made only on a 3-T scanner, and we cannot discuss the performance of this technique on scanners with another field strength. On the other hand, motion artefacts on 1.5-T scanners are usually less noticeable than on 3-T scanners, which allows us to speculate that this technique could be useful also for 1.5-T scanners. In addition, only 3 out of 17 patients under 7 years of age were scanned without anaesthetic support, which does not allow us to conclude on the possibility of scanning these patients without anaesthesia, but suggests that it can be possible. Further research is needed in patients under 7 years of age.
In conclusion, the FB-mTFE sequence allowed to obtain good quality T2* myocardial maps at 3-T for patients with iron overload in different groups of haematological diseases. Free-breathing scans with multiple signal accumulation technique did not require additional research packages and also facilitated and shortened the duration of the investigation in young children under anaesthetic support and in some cases allowed to even refuse it. This approach may pave the way for scanning patients who are unable to hold their breath for medical reasons without anaesthetic support. Further studies in patients of different age ranges are required.