Name: oxygenation-sensitive cardiac mri with vasoactive breathing maneuvers for the non-invasive assessment of coronary microvascular dysfunction
Uploaded: 2022-08-17T13:50:00
Description: Watch this Scientific Journal Video about Oxygenation-sensitive Cardiac MRI with Vasoactive Breathing Maneuvers for the Non-invasive Assessment of Coronary Microvascular Dysfunction at JoVE.com

This paper and methodology review was made possible by the entire team of the Courtois CMR Research Group at the McGill University Health Centre. Special thanks to our MRI technologists Maggie Leo and Sylvie Gelineau for the scanning of our participants and feedback on this manuscript.

All MRI scans utilizing OS-CMR with vasoactive breathing maneuvers should be performed in compliance with local institutional guidelines. The protocol outlined below has been used in studies approved by several&#160;institutional human research ethics committee. Written consent was obtained for all the human participant data and results described in this protocol and manuscript.
1. Broad overview
<ol>
	<li>Vary the inclusion and exclusion criteria depending on the study popu...

x`The addition of an OS-CMR acquisition with standardized, vasoactive breathing maneuvers to an already established research or clinical MRI protocol adds little time to the overall scan. With this short addition, information about underlying macro- and microvascular function can be obtained (Figure 2). An important consequence of endothelial dysfunction is the inability of the vasculature to respond to physiologic stimuli, as initially demonstrated through abnormal flow-mediated relaxation ...

This corrects the article <a href="https://dx.doi.org/10.3791/64149">10.3791/64149</a>

An erratum was issued for: <a href="https://www.jove.com/t/64149">Oxygenation-sensitive Cardiac MRI with Vasoactive Breathing Maneuvers for the Non-invasive Assessment of Coronary Microvascular Dysfunction</a>. The Abstract, Protocol, and Representative Results&#160;section were&#160;updated.

Erratum: Oxygenation-sensitive Cardiac MRI with Vasoactive Breathing Maneuvers for the Non-invasive Assessment of Coronary Microvascular Dysfunction

Oxygenation-sensitive cardiac magnetic resonance imaging (OS-CMR) uses the inherent paramagnetic properties of deoxyhemoglobin as an endogenous source of MR contrast1,2,3. Used in combination with standardized vasoactive breathing maneuvers (hyperventilation and apnea) as a potent non-pharmacologic vasomotor stimulus, OS-CMR can monitor changes in myocardial oxygenation as a marker for vascular function, thereby circumventing the need for any extrinsic, intravenous contrast or pharmacologic stress agents 4,5,6.
Breathing maneuvers, including breath-holds and hyperventilation, are highly effective vasoactive measures to alter vasomotion and, because of their safety and simplicity, are ideal for controlled endothelial-dependent vasomotion as part of a diagnostic procedure. Studies have shown an added effectiveness when combining hyperventilation with a subsequent breath-hold4,7, as during such a protocol, the vasoconstriction (through the associated decrease of blood carbon dioxide) is followed by vasodilation (increase of blood carbon dioxide); thus, a healthy vascular system transitions through the entire range from vasoconstriction to vasodilation with a strong increase in myocardial blood flow, which in turn increases myocardial oxygenation and, thus, the observable signal intensity in OS-CMR images. The use of cine images for the acquisition also allows for cardiac phase-resolved results with a better signal-to-noise ratio when compared to adenosine infusion8.
Breathing maneuvers can replace pharmacological stress agents for inducing vasoactive changes that can be used for assessing coronary vascular function. This not only reduces patient risk, logistical efforts, and associated costs but also helps in providing results that are clinically more meaningful. Pharmacologic stress agents such as adenosine trigger an endothelium-dependent response and, thus, reflect endothelial function itself. Such specific assessment of endothelial function so far was only possible by an intracoronary administration of acetylcholine as an endothelial-dependent vasodilator. This procedure, however, is highly invasive2,9 and, therefore, rarely performed.
Lacking access to direct biomarkers, several diagnostic techniques have used surrogate markers such as tissue uptake of an exogenous contrast agent. They are limited by the need for one or two intravenous access lines, contraindications such as severe kidney disease or atrioventricular block, and the need for the physical presence of staff with training in managing potentially severe side effects10,11. The most significant limitation of current imaging of coronary function, however, remains that myocardial perfusion as a surrogate marker does not reflect myocardial tissue oxygenation as the most important downstream consequence of vascular dysfunction2.
OS-CMR with vasoactive breathing maneuvers has been utilized to evaluate vascular function in numerous scenarios, including healthy individuals, macrovascular disease in patients with coronary artery disease (CAD), as well as microvascular dysfunction in patients with obstructive sleep apnea (OSA), ischemia with no obstructive coronary artery stenosis (INOCA), after heart transplantation, and heart failure with preserved ejection fraction (HFpEF)4,7,12,13,14,15,16. In a CAD population, the protocol for the breathing-induced myocardial oxygenation reserve (B-MORE) as derived from OS-CMR was proven to be safe, feasible, and sensitive in identifying an impaired oxygenation response in myocardial territories perfused by a coronary artery with a significant stenosis13.
In microvascular dysfunction, OS-CMR demonstrated a delayed myocardial oxygenation response in patients with obstructive sleep apnea, and a blunted B-MORE was found in patients with HFpEF and following heart transplantation12,14,16. In women with INOCA, the breathing maneuver led to an abnormally heterogeneous myocardial oxygenation response, highlighting the advantage of the high spatial resolution of OS-CMR15. This paper reviews the rationale and methodology for performing OS-CMR with vasoactive breathing maneuvers and discusses its clinical utility in the assessment of vascular pathophysiology in patient populations with microvascular dysfunction, specifically as they relate to endothelial dysfunction.
The physiological context of breathing-enhanced oxygenation-sensitive MRI 
Under normal physiologic conditions, an increase in oxygen demand is matched by an equivalent increase in oxygen supply through increased blood flow, resulting in no change in local deoxyhemoglobin concentration. In contrast, induced vasodilation leads to &#34;excess&#34; inflow of oxygenated blood without a change in oxygen demand. Consequently, more of the tissue hemoglobin is oxygenated, and thus, there is less deoxyhemoglobin, leading to a relative increase in OS-CMR signal intensity4,17. If vascular function is compromised, it cannot properly respond to an altered metabolic demand or stimulus to augment myocardial blood flow.
In the setting of a stimulus to elicit vasomotion, such as paced hyperventilation eliciting vasoconstriction or a long breath-hold eliciting carbon dioxide-mediated vasodilation, impaired vasomotor activity would result in a relative increase in local deoxyhemoglobin concentration compared with other regions, and, subsequently, a reduced change in OS-CMR signal intensity. In the setting of inducible ischemia, impaired vascular function would result in increased local demand not met by a local increase in myocardial blood flow even in the absence of epicardial coronary artery stenosis. In OS-CMR images, the net local increase in deoxyhemoglobin concentration leads to a decrease in local signal intensity2,18,19,20.
Attenuated vascular smooth muscle relaxation in response to endothelium-dependent and -independent vasodilators (including adenosine) has been demonstrated in patients with coronary microvascular dysfunction21,22,23,24,25,26,27. Endothelial-independent dysfunction is thought to be due to structural abnormalities from microvascular hypertrophy or surrounding myocardial pathology. In contrast, endothelial dysfunction results in both inadequate vasoconstriction and impaired (endothelium-dependent) vasorelaxation, typically caused by a loss of nitric oxide bioactivity in the vessel wall21,28. Endothelial dysfunction has been implicated in the pathogenesis of a number of cardiovascular diseases, including hypercholesterolemia, hypertension, diabetes, CAD, obstructive sleep apnea, INOCA, and HF23,24,28,29,30,31,32. In fact, endothelial dysfunction is the earliest manifestation of coronary atherosclerosis33. The imaging of endothelial function has very strong potential, given its role as a significant predictor of adverse cardiovascular events and long-term outcomes, with profound prognostic implications in cardiovascular disease states23,29,30,31,34,35.
In contrast to perfusion imaging, the breathing-induced myocardial oxygenation reserve (B-MORE), defined as the relative increase in myocardial oxygenation during a post-hyperventilation breath-hold allows for visualizing the consequences of such a vasoactive trigger on global or regional oxygenation itself2,36. As an accurate downstream marker of vascular function, B-MORE can, therefore, not only identify vascular dysfunction but also actual inducible ischemia, indicating a more severe local perfusion or oxygenation problem18,19,37. This is achieved through the ability of OS-CMR to visualize the relative decrease in deoxygenated hemoglobin, which is abundant in the capillary system of the myocardium, which itself represents a significant proportion of myocardial tissue24.
OS-CMR sequence 
The magnetic resonance imaging (MRI) sequence used for OS-CMR imaging is a prospectively gated, modified, balanced, steady-state, free precession (bSSFP) sequence acquired in two short-axis slices. This bSSFP sequence is a standard clinical sequence available (and modifiable) on all MRI scanners that perform cardiac MRI, making this technique vendor-agnostic and easily implemented. In a regular bSSFP cine sequence, echo time, repetition time, and flip angle are modified to sensitize the resulting signal intensity to the BOLD effect and, thus, create an oxygenation-sensitive sequence. This approach, a T2-prepared bSSFP readout, has previously been shown to be suitable for acquiring oxygenation-sensitive images with a higher signal-to-noise ratio, higher image quality, and faster scan times when compared to previous gradient echo techniques used for BOLD imaging38. Performing breathing-enhanced OS-CMR with this approach can be applied with very few, mild side effects (Table 1). Of note, more than 90% of participants complete this protocol with sufficiently long breath-hold times4,12,13,16.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>balanced SSFP MRI sequence</td>
			<td>Any</td>
			<td></td>
			<td>To modify to create the OS-CMR sequence</td>
		</tr>
		<tr>
			<td>DICOM/ Imaging Viewer</td>
			<td>Any</td>
			<td></td>
			<td>Best if the viewer has the ability for quantitative measurements (i.e., Area19 prototype software)</td>
		</tr>
		<tr>
			<td>Magnetic Resonance Imaging scanner</td>
			<td>Any</td>
			<td></td>
			<td>3 Tesla or 1.5 Tesla</td>
		</tr>
		<tr>
			<td>Metronome</td>
			<td>Any</td>
			<td></td>
			<td>Set to 30 breaths per minute. To use if manually communicating breathing maneuver instructions to participants.</td>
		</tr>
		<tr>
			<td>Speaker system</td>
			<td>Any</td>
			<td></td>
			<td>To communicate breathing maneuver instrucitons to participants through</td>
		</tr>
		<tr>
			<td>Stopwatch</td>
			<td>Any</td>
			<td></td>
			<td>To use if manually communicating breathing maneuver instructions to participants</td>
		</tr>
	</tbody>
</table>

oxygenation-sensitive cardiac mri with vasoactive breathing maneuvers for the non-invasive assessment of coronary microvascular dysfunction

Oxygenation-sensitive cardiac magnetic resonance imaging (OS-CMR) is a diagnostic technique that uses the inherent paramagnetic properties of deoxyhemoglobin as an endogenous source of tissue contrast. Used in combination with standardized vasoactive breathing maneuvers (hyperventilation and apnea) as a potent non-pharmacologic vasomotor stimulus, OS-CMR can monitor changes in myocardial oxygenation. Quantifying such changes during the cardiac cycle and throughout vasoactive maneuvers can provide markers for coronary macro- and microvascular function and thereby circumvent the need for any extrinsic, intravenous contrast or pharmacologic stress agents.
OS-CMR uses the well-known sensitivity of T2*-weighted images to blood oxygenation. Oxygenation-sensitive images can be acquired on any cardiac MRI scanner using a modified standard clinical steady-state free precession (SSFP) cine sequence, making this technique vendor-agnostic and easily implemented. As a vasoactive breathing maneuver, we apply a 4-min breathing protocol of 120 s of free breathing, 60 s of paced hyperventilation, followed by an expiratory breath-hold of at least 30 s. The regional and global response of myocardial tissue oxygenation to this maneuver can be assessed by tracking the signal intensity change. The change over the initial 30 s of the post-hyperventilation breath-hold, referred to as the breathing-induced myocardial oxygenation reserve (B-MORE) has been studied in healthy people and various pathologies. A detailed protocol for performing oxygen-sensitive CMR scans with vasoactive maneuvers is provided.
As demonstrated in patients with microvascular dysfunction in yet incompletely understood conditions, such as inducible ischemia with no obstructive coronary artery stenosis (INOCA), heart failure with preserved ejection fraction (HFpEF), or microvascular dysfunction after heart transplantation, this approach provides unique, clinically important, and complementary information on coronary vascular function.

Interpreting B-MORE 
In previously published studies utilizing OS-CMR with vasoactive breathing maneuvers, the global or regional B-MORE was calculated by comparing the first end-systolic image of the breath-hold to the end-systolic image closest to 15 s, 30 s, 45 s, etc. of the breath-hold. The end-systolic phase of the cardiac cycle was chosen for several reasons. The end-systolic image is the most consistent phase identified among and between readers: it contains the greatest number of pixels in ...

Watch this Scientific Journal Video about Oxygenation-sensitive Cardiac MRI with Vasoactive Breathing Maneuvers for the Non-invasive Assessment of Coronary Microvascular Dysfunction at JoVE.com

Oxygenation-sensitive Cardiac MRI with Vasoactive Breathing Maneuvers for the Non-invasive Assessment of Coronary Microvascular Dysfunction

The assessment of microvascular function by oxygenation-sensitive cardiac magnetic resonance imaging in combination with vasoactive breathing maneuvers is unique in its ability to assess rapid dynamic changes in myocardial oxygenation in vivo and, thus, may serve as a critically important diagnostic technique for coronary vascular function.

Oxygenation-sensitive cardiac magnetic resonance imaging (OS-CMR) is a diagnostic technique that uses the inherent paramagnetic properties of ...

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