Ischaemic Heart Disease

2D echocardiography

This allows imaging of the heart and great vessels through a limited chest wall acoustic window ▸ transoesophageal echocardiography can also image the heart

Doppler echocardiography

This can estimate blood flow and pressure gradients

Contrast medium echocardiography

This uses IV microbubble contrast ▸ it improves the definition of the margins of the ventricular cavity and enhances the accuracy of echocardiography for assessing ventricular function in poor echocardiography patients ▸ it can also assess myocardial perfusion

Stress echocardiography

• This detects reversible wall motion abnormalities (indicating reversible ischaemia) using the same stimulants as for CT or MR stress imaging

• It enables risk stratification for patients with known or suspected IHD (a normal study indicates a very good prognosis)

• It enables preoperative risk assessment (this determines myocardial viability)

• It is more sensitive for detecting ischaemia than a conventional ECG-based exercise test ▸ the endpoint of the test is the appearance of new wall motion abnormalities (chronic ischaemia causes diffuse dysfunction, an acute myocardial infarction causes more localized changes)

• Factors affecting the accuracy of the technique: the threshold for defining a significant stenosis ▸ whether there is single or multivessel disease ▸ whether an adequate stress is achieved (especially for exercise-based protocols) ▸ the presence of other disease processes which affect myocardial function (such as cardiomyopathy, microvascular disease, or hypertrophy)

Spin-echo imaging

‘Black blood’ imaging: the myocardium and vascular wall appear bright and the blood dark ▸ useful for high spatial resolution static anatomical imaging (but rather slow to perform)

• 180° inverting pulse applied to the imaging volume followed by a slice selective 180° ‘de-inversion’ pulse: net result is inverted spins outside the imaged slice but spins within the imaged slice are unchanged (they have experienced both inversion and de-inversion) ▸ the time at which this is applied is the time at which the longitudinal magnetization of blood has reached zero from the initial inversion – therefore blood flowing into the slice will generate no signal (‘black blood’) ▸ variable signal from slow or in-plane blood flow (producing artefacts)

Gradient-echo imaging

• ‘Bright blood’ imaging: blood produces a higher SI than that seen with spin-echo sequences

• This is because only one radiofrequency pulse is used and there is less time for the blood to move out of the imaging plane between slice selection and image acquisition (turbulent blood flow will produce areas of signal loss)

• The gradient-echo sequence can be repeated more rapidly with a reduced RF flip angle, allowing the acquisition of cine loops ▸ this also allows quantification of ventricular stroke volumes (comparing end-systolic and end-diastolic volumes)

Phase shift velocity mapping

• This allows quantification of flow velocities

• Positive velocity encoding: if a gradient is applied in the direction of blood flow for a finite time and then turned off, the relationship of phase will change in relation to the two ends of the gradient (the protons at the stronger end of the gradient precess at a faster rate during its application than those at the weaker end)

• Negative velocity encoding: when the gradient is turned off, the rate of precession becomes constant again in the slice, but the phase relationship has changed and the phase signature remains ▸ when the gradient is reversed and applied for the same period of time as the original gradient, the phase signature of still material is cancelled, but flowing blood moving during the gradients to a different phase territory retains a phase change proportional to its velocity

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    Simple subtraction of the images obtained with positive and negative velocity encoding will generate a phase image whose pixel value is directly proportional to the blood flow velocity

Myocardial perfusion imaging

• T1WI: the heart is imaged during the bolus administration of a gadolinium contrast agent

• Ischaemic areas will demonstrate delayed enhancement

Contrast-enhanced MRA

• T1WI: accurately determines CABG patency-possibly that of the infarct-related artery after coronary thrombolysis

• It has an equivalent accuracy to MDCTA (however MDCTA is quicker and easier to use)

Myocardial tagging

Prior to a GE cine loop a special selective excitation pulse produces a 2D grid structure within the myocardium (narrow planes of very low SI appearing as a latticework)

• This moves and deforms throughout the cardiac cycle allowing assessment of movement

Cardiac gating

• Prospective gating: the QRS complex of the ECG triggers the imaging such that the image is formed from a specific part of the cardiac cycle

• Retrospective gating: the imaging sequence is repeated continuously with a constant repeat time while the ECG is monitored ▸ the data are sorted at the completion of the sequence and adjusted to give images at specific parts of the cardiac cycle

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    Prevention of respiratory motion (breath holding) is important

CMR imaging planes

Usually obtained in orientation to the axes of the heart (as for echocardiography)