In this study, we found that systolic T1 mapping could be obtained from diastolic T1 mapping and cine MRI images, there was little change in myocardial T1 value due to the cardiac cycle, and the systolic right ventricular myocardial T1 value was high in patients with right ventricular overload.
Under standard settings, the cardiac cycle is determined from the QRS complex, and T1 mapping is performed during diastole [9–10]. Therefore, systolic imaging should be devised, as in this study. Our results suggest almost no difference in T1 mapping processing time between individuals, and systolic T1 mapping was possible by subtracting 185 msec from the systolic time of the cine MRI images. It is necessary to examine whether this value of 185 msec remains the same for other MRI devices in the future. Once the processing time for T1 mapping is obtained at one's facility, systolic T1 mapping can always be obtained from cine MRI images, which is very useful. When setting the systolic time, Ursula et al. examined systolic T1 mapping by shifting a certain amount of time from diastole [11]. In that method, changes in heart rate cause a shift in systolic timing. Therefore, our method is better at setting systolic timing independent of heart rate.
In addition, we found that the myocardial T1 value hardly changed during the cardiac cycle. Carlo et al. reported no difference in left ventricular myocardial T1 values between diastole and systole [12]. In contrast, Ursula et al. reported that the myocardial T1 value significantly decreased during systole than during diastole, but the difference was approximately 2.5%, which is unlikely to be clinically significant [11]. The slight change in myocardial T1 value with the cardiac cycle indicates no problem in evaluating the right ventricular myocardial T1 value in either the diastolic or systolic cardiac cycle. In determining the normal right ventricular myocardial T1 value, it is difficult to measure the diastolic right ventricular myocardial T1 value in healthy subjects. Because the right ventricular myocardium has a thin myocardial wall and rough endocardial surface, the myocardium is insufficiently thick to surround the myocardial ROI and is susceptible to blood-induced T1 elevation. Because the myocardial wall is thicker during systole than diastole, the free and inferior walls of the right ventricle increase the number of sites where the ROI can be obtained. Despite this, the measurement locations are often limited. When considering setting normal values, it is important to take multiple cross sections to increase the number of measurable locations as we did.
Regarding the site of myocardial T1 measurement, our results showed a significant difference between the left ventricular septal and posterior walls, consistent with a previous report [5]. The myocardial T1 value of the left ventricular posterior wall is known to be lower than that of the septal wall owing to the effects of cardiac motion [5]. Similar results were expected for the right ventricular myocardium. However, there was no significant difference in myocardial T1 values between the right ventricular free and inferior walls during systole in healthy subjects. Therefore, if it is difficult to obtain the myocardial T1 value on the free wall, there is no significant problem in evaluating the myocardial T1 value on the inferior wall. It is desirable to evaluate the same location over time.
Consistent with previous reports, patients with right ventricular myocardial injury had elevated right ventricular myocardial T1 values [13–14]. The right ventricular myocardial T1 value is known to be elevated in patients with pulmonary hypertension and after surgery for tetralogy of Fallot. But, in previous reports, the right ventricular myocardial T1 value was evaluated during diastole [13–14]. The validity of the diastolic right ventricular myocardial T1 value has been cited as a limitation in all the previous reports. Among patients with right ventricular overload, especially those with right ventricular pressure overload, the thickness of the right ventricular myocardium is sufficient even during diastole due to right ventricular hypertrophy. Therefore, measurement of the myocardial T1 value is almost unaffected by blood and can be measured accurately. As mentioned earlier, it is challenging to measure the normal T1 value in diastolic right ventricular myocardium. Without knowing normal values, it is not possible to evaluate right ventricular T1 values in disease groups. Even in patients with a thick right ventricular myocardium, it is possible to compare with normal values by imaging systolic T1 mapping and quantitative evaluation of right ventricular myocardial damage can now be performed.
One of the limitations of this study is the small number of cases. In particular, there are limited data on healthy subjects. Since myocardial T1 changes are known to depend on sex and age, it is necessary to increase the number of cases in the future and determine the normal value of right ventricular myocardial T1 during systole. Also, due to the small number of cases, we were unable to separate patient groups into pressure or volume overload. It is necessary to study with a large number of patients in the future. Second, it is difficult to measure the right ventricular myocardial T1 value. The right ventricular myocardium of healthy subjects was measured as a normal myocardium at any site. There might be a difference depending on the site in the patient group. To ensure sufficient thickness of the right ventricular myocardium, even during systole, it is considered appropriate to evaluate systolic T1 mapping only in patients with right ventricular overload, and further investigation is required.
We quantitatively evaluated the properties of the right ventricular myocardium by right ventricular myocardial T1 mapping during systole. The right ventricular myocardial T1 value can be evaluated by measuring it during systole rather than diastole and can be useful for evaluating myocardial damage.