Abstract
Background
Myocardial oxygenation imaging could help determine the presence of microvascular dysfunction associated with increased cardiovascular risk. However, it is challenging to depict the potentially small oxygenation alterations with current noninvasive cardiac MRI blood oxygen level–dependent (BOLD) techniques.
Purpose
To demonstrate the cardiac application of a gradient-echo spin-echo (GESE) echo-planar imaging sequence for dynamic and quantitative heartbeat-to-heartbeat BOLD MRI and evaluate the sequence in populations both healthy and with hypertension in combination with a breath hold–induced CO2 intervention.
Materials and Methods
GESE echo-planar imaging sequence was performed in 18 healthy participants and in eight prospectively recruited participants with hypertension on a 3.0-T MRI system. T2 and T2* maps were calculated per heartbeat with a four-parameter fitting technique. Septal regions of interests were used to determine T2 and T2* values per heartbeat and examined over the course of a breath hold to determine BOLD changes. T2 and T2* changes of healthy participants and participants with hypertension were compared by using a nonparametric Mann-Whitney test.
Results
GESE echo-planar imaging approach gave spatially stable T2 and T2* maps per heartbeat for healthy participants and participants with hypertension, with mean T2 values of 43 msec ± 5 (standard deviation) and 46 msec ± 9, respectively, and mean T2* values of 28 msec ± 5 and 22 msec ± 5, respectively. The healthy participants exhibited increasing T2 and T2* values over the course of a breath hold with a mean positive slope of 0.2 msec per heartbeat ± 0.1 for T2 and 0.2 msec per heartbeat ± 0.1 for T2*, whereas for participants with hypertension these dynamic T2 and T2* values had a mean negative slope of −0.2 msec per heartbeat ± 0.2 for T2 and −0.1 msec per heartbeat ± 0.2 for T2*. The difference in these mean slopes between healthy participants and participants with hypertension was significant for both T2 (P < .001) and T2* (P < .001).
Conclusion
Gradient-echo spin-echo echo-planar imaging sequence provided quantitative T2 and T2* maps per heartbeat and enabled dynamic heartbeat-to-heartbeat blood oxygen level–dependent (BOLD)-response imaging by analyzing changes in T2 and T2* over the time of a breath-hold intervention. This approach could identify differences in the BOLD response between healthy participants and participants with hypertension.
© RSNA, 2020
Online supplemental material is available for this article.
See also the editorial by Friedrich in this issue.
Summary
Dynamic quantitative cardiac blood oxygen level–dependent MRI with a multiecho gradient-echo spin-echo sequence enabled detection of oxygenation differences during a breath hold between healthy control participants and participants with hypertension.
Key Results
■ A multiecho gradient-echo spin-echo with echo-planar imaging readout enabled acquisition of five echoes per heartbeat, which provided simultaneous quantitative T2 and T2* maps per heartbeat.
■ During a standard breath hold, increasing T2 and T2* values of 0.2 msec per heartbeat were observed for healthy participants, which can be interpreted as a blood oxygen level–dependent (BOLD) response.
■ During a standard breath hold, decreasing T2 values of −0.2 msec per heartbeat (P < .001) and T2* values of −0.1 msec per heartbeat (P < .001) were observed for participants with hypertension compared with healthy participants, which indicated a compromised BOLD response.
Introduction
Cardiovascular diseases are often associated with a disruption of the oxygen demand and supply equilibrium, which can lead to functional impairments and heart failure (1). There are numerous methods to diagnose myocardial ischemia by using surrogate markers (2), but these techniques often need contrast agents, vasodilators, or radiation, and do not directly reflect the ischemic response (3).
Cardiac MRI can be used to assess myocardial oxygenation by using the blood oxygen level–dependent (BOLD) effect (4). Both T2- and T2*-weighted imaging and mapping approaches (5–7) have been used as cardiac BOLD MRI techniques to help identify coronary artery disease without the use of exogenous contrast agent (8–11). Breath-hold interventions are recognized to trigger a cardiac BOLD response (8,11–13), causing an increase in vascular CO2 levels and myocardial vasodilation within 15 seconds (14), which results in an increase of myocardial T2 and T2*. In addition to depicting coronary artery disease, it has been hypothesized that cardiac BOLD MRI may be able to depict microvascular dysfunction in conditions such as hypertension (1,15) because of expected differences in vascular response (16). Depiction of these relatively subtle differences by using current BOLD MRI techniques is challenging, but several approaches have been developed to increase this feasibility. For example, signal-to-noise ratio can be improved by averaging over multiple heartbeats or breath holds (6,9,10), or can further be preserved while increasing the spatial resolution by introducing compressed sensing and subspace modeling (17). Furthermore, to improve BOLD sensitivity, mapping and increasing the field strength have been introduced (7,17,18).
We investigated a multiecho gradient-echo spin-echo (GESE) sequence to evaluate the feasibility of quantitative T2 and T2* mapping per heartbeat for depicting microvascular dysfunction with sufficient signal-to-noise ratio, spatial resolution, and sensitivity to BOLD changes because of breath-hold interventions. Quantitative and temporal improvements have been accomplished for BOLD MRI in the brain by using GESE with echo-planar imaging readouts (19,20). Combining these readouts into T2 and T2* maps helps to correct for large vessel contributions to the oxygenation maps (19,21). When applied to the heart (6,22), this mapping approach mitigates the influence of heart rate changes during an intervention and offers quantitative myocardial BOLD readouts, allowing for the depiction of dynamic BOLD response changes.
The purpose of our study was to demonstrate the cardiac dynamic application of GESE echo-planar imaging for quantitative heartbeat-to-heartbeat BOLD response imaging. The sequence was tested in healthy volunteers while executing a breath hold to induce a CO2-triggered intervention (12,23) and evaluated in patients with hypertension to explore its clinical applicability.
Materials and Methods
This study was approved by the institutional review board at two sites (2015P002346 and 2016P00674) and written informed consent was obtained from all participants. The U.S. study site was Health Insurance Portability and Accountability Act compliant.
Study Participants
MRI was performed in 18 healthy participants and nine participants with hypertension who were prospectively recruited. Other than the inclusion criteria of age 18–45 years and no previous cardiac events or smoking, the healthy participants also had to meet the criteria of body mass index of 25 kg/m2 or less, whereas the population with hypertension required clinically diagnosed hypertension with treatment, a body mass index of 30 kg/m2 or greater, and an increased blood pressure of 120/80 mm Hg or greater measured after the participant underwent MRI (Table). Six healthy participants underwent repeat imaging within 30 days to obtain intrasubject reproducibility.
Participant Demographics and MRI Parameters
MRI Acquisition
All examinations were performed by using a 3.0-T MRI system (Skyra or Prisma; Siemens Healthcare, Erlangen, Germany). GESE echo-planar imaging (Fig 1, A) was performed at the diastolic phase during an end-expiration breath hold of a maximum of 30 data repetitions for all participants in addition to a parallel imaging calibration acquisition and four dummy repetitions to reach a steady state (20). In the same midventricular slice as GESE echo-planar imaging, clinical standard T2 and T2* mapping protocols were performed in part of both populations for direct comparison of the T2 and T2* values. Conventional cine and late gadolinium chelate–enhanced examinations were performed to rule out the presence of focal ischemia or change in ejection fraction in the participants with hypertension. Additional details are in Appendix E1 (online).
Figure 1:
The sequence is designed on the basis of, A, gradient-echo (GE) spin-echo (SE) echo-planar imaging sequence with five echoes, of which two gradient-echo acquisitions are placed before (first echo time [TE], 9.8 msec; and second TE, 23.6 msec) and three mixed SE acquisitions (third TE, 37.8 msec; fourth TE, 51.9 msec; and fifth TE, 66.1 msec) are placed after the 180 radiofrequency pulse. The trigger delay (t-delay) starts at the cardiac R peak at start of the systolic phase and delays the acquisition until the middiastolic phase. B, Raw images for each TE acquired during a single heartbeat are shown. t-delay = trigger delay, RF = radiofrequency.
GESE Echo-planar Imaging BOLD Analysis
Data quality assessment of the gradient-echo and mixed spin-echo images from GESE echo-planar imaging was performed by calculating the myocardial signal-to-noise ratio in the septum. Semiautomatic segmentation of the endo- and epicardium was performed before mapping. T2- and T2* maps were calculated from GESE echo-planar images per heartbeat by using a multiparametric voxel-by-voxel fitting approach. The mean T2 and T2* values in a septal region of interest acquired per heartbeat were used for further comparison. The changes in the septal T2 and T2* values in msec per heartbeat over the course of a breath hold were determined by linear regression during the acquired heartbeats and interpreted as the BOLD response to the breath hold. Furthermore, the percentage change of T2 and T2* values during 20 heartbeats was calculated by using the y-axis intersect and slope of the linear fit. More analysis details can be found in Appendix E1 (online).
Statistical Analysis
The mean T2 and T2* values from the first 10 heartbeats (Appendix E1 [online]) of a septal region of interest in the GESE echo-planar imaging−based maps were compared with the standard T2 and T2* maps by using a paired Student t test. Participant demographics and static T2 and T2* values between populations were compared by using an unpaired Student t test. Linear regression analysis of the T2 and T2* values during a breath hold were evaluated by testing the null hypothesis that the slopes are 0 by an F test and reporting the R2 and 95% confidence intervals of the fit. Reproducibility of this slope was tested by a Bland-Altman bias assessment, reporting the standard deviation of the bias and the 95% confidence intervals. The T2 and T2* slopes, or BOLD response, were compared between the healthy participants and participants with hypertension by using a nonparametric Mann-Whitney test. A nonparametric Spearman correlation was used to identify a possible relationship between heart-rate changes and T2 and T2* slopes. All statistical analyses were performed by using statistical software (GraphPad Prism, version 8; GraphPad Software, La Jolla, Calif). P values less than .05 indicated statistical significance.
Results
Study Participants
Participant demographics are in the Table, which only showed difference in body mass index (P = .001) between the healthy participants and participants with hypertension. One participant with hypertension was excluded after cardiovascular MRI because no increased blood pressure was recorded, resulting in eight participants with hypertension who were in our study. The ejection fractions of the participants with hypertension were within normal range and no scar tissue was present (Table) (24).
Image Acquisition
The mean signal-to-noise ratio for each echo time acquired at GESE echo-planar imaging was 26 ± 5 (standard deviation), 16 ± 4, 9 ± 1, 7 ± 1, and 7 ± 1, from shortest to longest echo times (Fig 1, B). The voxel-by-voxel fitted T2 and T2* maps show low spatial variation across the myocardium (Fig 2), although dephasing artifacts can be seen in large B0 variation voxels along the epi- and endocardium, as expected.
Figure 2:
An example of the T2 and T2* maps in, A, a healthy female participant (age, 25 years) and, B, a male participant with hypertension (age, 38 years) acquired from a single heartbeat. The smoothed maps are found by using a local 3 × 3 Gaussian filter and are only used for visualization; raw maps are used for all analysis. Root mean square error (RMSE) maps of the T2 and T2* fits per voxel are shown; voxels with a RMSE greater than 5% are excluded from analysis. A region of interest in the septum was used for blood oxygen level–dependent analysis. The mean septal T2 and T2* values are plotted per heartbeat over the time of a breath hold, with a linear regression fit through these measures.
The mean T2 and T2* values from GESE echo-planar imaging in the septal region of interest across healthy participants were 43 msec ± 5 and 28 msec ± 5, respectively, and 46 msec ± 9 and 22 msec ± 5 in the participants with hypertension (Table). These GESE echo-planar imaging−based T2 and T2* values were comparable with the standard T2 mapping values of 42 msec ± 6 (P = .66) and T2* mapping values of 24 msec ± 3 (P = .17). The standard T2 value of 40 msec ± 3 in participants with hypertension was slightly lower than the GESE echo-planar imaging−based T2 values (P = .11) (Table).
BOLD Effect in Healthy Participants and Participants with Hypertension
In healthy participants, the septal region of interest showed increasing T2 and T2* values at breath hold, with a mean slope of 0.2 msec per heartbeat ± 0.1 for T2 and 0.2 msec per heartbeat ± 0.1 for T2* (Table). Example T2 and T2* maps of a single heartbeat in a representative participant are shown in Figure 2, A, with corresponding heartbeat-to-heartbeat measurements, showing a positive slope during the time of a breath hold for both T2 (P = .01) and T2* values (P < .001).
In participants with hypertension, decreasing T2 and T2* values were observed during the breath hold, with a mean slope of −0.2 msec per heartbeat ± 0.2 for T2 and −0.1 msec per heartbeat ± 0.2 for T2* (Table). Figure 2, B, shows the T2 and T2* maps and linear fit of a representative participant with hypertension with a negative slope for both T2 (P = .02) and T2* values (P < .001).
The Bland-Altman plots show the T2 and T2* slopes in healthy participants were reproducible between imaging sessions with a bias of 0 msec per heartbeat ± 0.02 and 0.03 msec per heartbeat ± 0.02, respectively (Figs 3, E1 [online]). Furthermore, there was no correlation between the T2 and T2* slopes and the heart rate changing over the course of a breath hold (P = .55 and P = .87, respectively), suggesting these results are heart-rate independent.
Figure 3:
Bland-Altman plots of the average T2 (left) and T2* (right) slopes in six healthy participants over the time of a breath hold by comparing the difference between these slopes between the first examination and the second examination separated by an average of 2.4 days ± 4.7 (standard deviation [SD]). The T2 slopes have a mean bias of 0 msec per heartbeat (ms/hb) ± 0.02 and the T2* slopes have a mean bias of 0.03 msec per heartbeat ± 0.02.
The fitted T2 and T2* slopes for all participants are shown in Figure 4, A, which shows difference between mean T2 slopes (P < .001) and mean T2* slopes (P < .001) in healthy participants and participants with hypertension, indicating a different BOLD response to the breath hold. The percentage change in T2 and T2* over the first 20 heartbeats are shown in Figure 4, B. All healthy participants exhibited positive T2 and T2* changes, whereas all but one participant with hypertension showed negative T2 and T2* changes, suggesting that in this participant, despite having hypertension, the BOLD response to the breath hold was in line with healthy participants.
Figure 4:
Box-and-whisker plots for T2 and T2* slopes and percentage change. A, T2 and T2* slopes are plotted for the 18 healthy participants with a mean slope of 0.2 msec per heartbeat (ms/hb) ± 0.1 (standard deviation) for T2 and 0.2 msec per heartbeat ± 0.1 for T2*. T2 and T2* slopes are plotted for the eight participants with hypertension and have a mean slope of −0.2 msec per heartbeat ± 0.2 for T2 (P < .001) and −0.1 msec per heartbeat ± 0.2 for T2* (P < .001), which are both different healthy participants despite the one participant with hypertension who showed positive healthy T2 and T2* slopes. B, T2 and T2* percentage changes over 20 heartbeats are plotted for healthy participants, with a mean increase of 10.1% ± 5.0 and 14.9% ± 10.2 of T2 and T2*, respectively. T2 and T2* percentage changes over 20 heartbeats are plotted for the participants with hypertension and have a mean decrease of −6.8% ± 8.8 (P < .001) and −7.1% ± 15.0 (P < .001) for T2 and T2*, respectively, compared with healthy participants.
Time-series data from participants with the lowest, mean, and highest T2 and T2* slopes in both groups are shown in Figure 5. All healthy participants had positive slopes for T2 and T2* that were significantly different from 0 (Fig 5, A), whereas in the participants with hypertension these slopes were 0 or negative and significantly different from 0 (Figs 5, B, E2 [online]).
Figure 5:
Graphs depict T2 and T2* values over the time of a breath hold. A, T2 and T2* values over time in three representative healthy participants; all values have a significant positive slope because of blood oxygen level–dependent (BOLD) change during breath hold. B, The T2 and T2* slopes in three representative participants with hypertension displaying a compromised BOLD change during breath hold. The T2 and T2* slopes of the remaining participants with hypertension can be found in Appendix E1 (online).
Discussion
Quantitative myocardial blood oxygen level–dependent (BOLD) readouts may allow for the depiction of subtle dynamic BOLD response changes in populations at increased cardiovascular risk, such as hypertension. Gradient-echo spin-echo (GESE) echo-planar imaging is a dynamic approach to noninvasively image myocardial BOLD changes over the course of a standard breath hold with direct quantitative T2 and T2* measures. The mean increasing T2 and T2* slopes of 0.2 msec per heartbeat ± 0.1 in healthy participants is concordant with previously published results (8,12,13) regarding breath-hold intervention but without the need for breathing maneuvers, averaging over multiple heartbeats, or sensitivity to heart-rate changes.
Patients with hypertension are known to undergo alterations in microvascular resistance (1,25), and the difference in T2 slopes of −0.2 msec per heartbeat ± 0.2 (P < .001) and T2* slopes of −0.1 msec per heartbeat ± 0.2 (P < .001) compared with healthy participants indicate that the BOLD response from breath hold is compromised, possibly because of a slower or nonexistent vascular response to the increase in CO2 (14,25). The sensitivity of this dynamic cardiac BOLD MRI approach by using only a breath hold could be achieved by allowing analysis of the rate of T2 and T2* changes instead of static comparisons between rest and stress states (5–7,10,11,15). Interestingly, another cardiac BOLD MRI technique evaluated in participants with coronary artery disease, in whom 54% had hypertension, also showed a weaker BOLD response in the remote areas compared with healthy control participants (8), which is similar to the GESE echo-planar imaging results in our study.
Although the differences in slopes between healthy participants and participants with hypertension are small, this quantitative and dynamic GESE echo-planar imaging approach is able to depict these subtle changes because of its combination of dynamic and quantitative abilities. This creates opportunities in the context of population and longitudinal studies to investigate subtle BOLD response changes caused by cardiovascular diseases but a broader populational study is needed to determine diagnostic abilities of this technique. Furthermore, other populations, such as patients with diabetes or hypercholesterolemia, are expected to have a similarly compromised vascular response, and this warrants further study (25).
Despite the promising differences of T2 and T2* slopes between healthy participants and participants with hypertension, the GESE echo-planar imaging BOLD cardiovascular MRI technique has some limitations that constrain its wide clinical applicability. Whereas the T2* maps from GESE echo-planar imaging are sensitive to BOLD changes, the maps may have dephasing artifacts on the lateral wall (6) and can be affected by lung volume changes because of B0 inhomogeneity. To mitigate these T2*-based limitations, this study restricted the acquisitions to end-expiration to reduce B0 fluctuation throughout the breath hold. Also, it constrained the BOLD analysis to a septal region of interest, which is a representative of the whole myocardium in diffuse microvascular diseases such as hypertension (25). However, this approach would be less applicable in spatially varying coronary artery disease, such as circumflex occlusion. Possible solutions to this limitation include incorporating spiral or radial readouts or moving the acquisitions to a lower field strength, although this may result in lower sensitivity to BOLD changes (18). Furthermore, T2 maps are more specific to macrovascular contribution and are less sensitive to the dephasing artifacts but have a lower signal-to-noise ratio (21) that could compromise the interpretation of the subtle BOLD changes. Consequently, the combination of both T2 and T2* maps offers complementary information for the assessment of BOLD in microvascular diseases.
Our protocol was somewhat limited regarding the image encoding that could fit in the diastolic phase. A relatively large voxel size helped to maintain reasonable encoding times and signal-to-noise ratio but increased the risk of obtaining compromised T2 and T2* maps because of partial volume averaging effects. The number of echoes was limited to five, which may have compromised the accuracy of the T2 and T2* maps.
In addition to our study, it would be of interest to compare GESE echo-planar imaging BOLD MRI with other existing cardiac BOLD MRI techniques. Previous approaches have used hyperventilation followed by an extended breath hold (8,12), which enhances the detectable BOLD effect by increasing the coronary vasomotion range. The GESE echo-planar imaging BOLD MRI technique could also be applied in combination with these breathing maneuvers for a direct comparison to existing techniques.
In conclusion, the gradient-echo spin-echo echo-planar imaging sequence provided quantitative T2 and T2* maps per heartbeat and enabled dynamic heartbeat-to-heartbeat imaging of blood oxygen level–dependent (BOLD) changes due to breath-hold intervention. This approach has the potential to contribute to the understanding of microvascular diseases. Further research in populations with hypertension and other populations at increased cardiovascular risk (16,25) should be performed to explore the actual diagnostic value of this BOLD MRI approach.
APPENDIX
SUPPLEMENTAL FIGURES
K.S. supported by the National Institutes of Health (R01EB020613). G.J.H.S. and N.H.J.P. supported by the Dutch Heart Association (2016T042).
Disclosures of Conflicts of Interest: M.v.d.B. disclosed no relevant relationships. M.K.M. disclosed no relevant relationships. G.J.H.S. disclosed no relevant relationships. S.H.H. disclosed no relevant relationships. K.E.E. Activities related to the present article: disclosed grant from the South-Eastern Norway Regional Health Authority. Activities not related to the present article: disclosed no relevant relationships. Other relationships: disclosed no relevant relationships. R.H.J.A.S. disclosed no relevant relationships. D.E.S. disclosed no relevant relationships. C.C. disclosed no relevant relationships. B.R.R. disclosed no relevant relationships. N.H.J.P. disclosed no relevant relationships. C.T.N. disclosed no relevant relationships. R.J.H.B. disclosed no relevant relationships. K.S. disclosed no relevant relationships.
Abbreviations:
- BOLD
- blood oxygen level dependent
- GESE
- gradient-echo spin echo
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