Under certain circumstances, it may not be technically feasible to use an ECG waveform for gated cardiac acquisitions. For example, irregular heart rates or low ECG voltages can render the ECG waveform unusable. In other circumstances, a “quick and dirty” waveform for a gated MR angiography sequence may be sufficient. Under these conditions, a peripheral photo-pulse waveform can be used as an ECG substitute.

Blood flowing in the capillaries of peripheral tissues (fingers or toes) can be measured by photo-plethysmography. This technology involves applying a light beam (typically within the infrared region of the electromagnetic spectrum) and detecting the light reflected or back-scattered from the beam as it passes through the epidermal and dermal layers of the
skin. Changes in blood flow in the capillaries modify the optical path length and, hence, the amount of back-scattered light. Typical photo-pulse sensors use independent transmitting and receiving detectors separated by a small distance, which allows simultaneous transmission and detection of the beam. Because the electrical activity of the heart precedes
the flow of blood outside of the left ventricle, the waveform has an inherent delay from the peak of the R wave of the QRS complex to the peak of the flow profile. For this reason, peripheral photo-pulse triggering is used less commonly than ECG-based methods.
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The first heart’s electrical activity was first measured in 1889 by Augustus Desiré Waller, who measured the electrical potential difference (voltage) across electrode pairs placed at five separate anatomical locations (two on the arms, two on the legs, and one at the mouth). Waller’s method of measuring the time-varying voltage signals across various electrode pairs (lead combinations) is commonly referred to as the electrocardiogram (ECG). The original configuration suggested by Waller was modified by Willem Einthoven in 1908.
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ECG (electrocardiogram) and MRI (magnetic resonance image) what were the connection between these two’s? ECG gating is a fundamental part of a cardiac MRI examination. Some reasons of ECG gating on MRI’s are below:
? To enable consistent temporal sampling of data throughout the cardiac cycle, which is necessary for cine acquisitions.
? To allow acquisition of static images at a given phase of the cardiac cycle.
? For correct sorting of static and dynamic k-space data obtained over multiple R-R intervals using segmented acquisition schemes.
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Conceptually, a cardiac MRI pulse sequence can be broken down into five constituent categories (boxes 1-5 below). Each category contains multiple components that can be combined in various ways to provide the full range of image contrasts and applications (eg, studies of function or morphology). The scheme below shows these five categories and the resultant image (box 6), as defined by the blood pool signal. This section describes each category and gives a list of components that make up each one.

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