Magnetic Resonance Imaging Notes: Respiratory and Cardiac Gating

Respiratory Gating

- the simplest method of eliminating respiratory motion artifact is to suspend respiration entirely though breath hold
- respiratory gating and respiratory compensation were utilized because not all patients can hold their breath long enough to acquire adequate images

Respiratory Compensation

- also known as exorcist, ROPE and COPE
- performed through reordered phase encoding
- throughout the respiratory cycle, anterior chest wall motion is monitored with pressure transducer that surrounds patient’s chest
- phase encoding steps are then reordered to decrease the intensity of respiratory motion artifacts and changes location of the motion artifacts within the data set
- has potential advantage over respiratory gating because acquisition tiem is not increased
- signal averaging results involved results in marked resolution loss and obscures fine detail
- a complicated real-time technique

Respiratory Gating

- MRI with respiratory gating is a simple and practical approach to reduce respiratory motion effects
- Data are collecting during continuous breathing but are used for image reconstruction only if they are collected within a reference range
- Before, reference for respiratory motion was obtained by placing a belt containing a displacement transducer around the upper abdomen
- Recently, navigation spin echo has been used to monitor diaphragmatic motion
- Acceptance or rejection of data can be made in real-time or retrospectively
- Disadvantage is increased imaging time

Cardiac Gating

- Used to reduce cardiac motion and can be monitored by measuring the ECG signal via placement of MRI-compatible electrodes on either the patient’s chest or back
- It is thought that placement of the electrodes dorsally may reduce artifact caused by lead motion
- If leads crossed out each other or looped, it can cause induction of undesired currents and the possibility of surface burns
- R-R interval is measured and image acquisition is triggered by the R-wave
- Alternative method is peripheral gating or plethysmography, where gating is triggered by the peripheral pulse via a small probe placed at the fingertip

Coils

- Standard gradient body coil and the phased array surface coil are the two coils most commonly used in thoracic imaging
- Phased array coil provide good central and peripheral imaging, maintain field homogeneity and improve SNR above that of the standard body coil
- Smaller flexible surface coils, and sometimes dedicated shoulder coils, are also used for imaging the superior sulcus and brachial plexus

Contrast Agents

- Intravenously administered gadolinium chelates is the most commonly use CM for thoracic MRI
- CM used in MRI are all paramagnetic agents that increase signal and have relaxivity rates
- Paramagnetic agents are administered prior to T1-wieghted image acquisition
- The one exception to this in the thoracic are double dose dynamic gadolinium-enhanced sequences
- Prior to administration of CM, patient’s medical history, specifically drug allergies must be evaluated

Specific uses

1. Aorta and Great Vessels
- MRI has become an important method of assessing dissection, aneurysms, pseudoaneurysms and congenital anomalies
- T1-weighted spin echo imaging with intravoxel dephasing is the most useful sequence for imaging the thoracic aorta and great vessels
- A newer method for MRI of the aorta and great vessels is dynamics double dose gadolinium-enhanced 3D imaging
- Injection is carefully timed to obtain images during peak bolus as determined by the initial test dose

2. Cardiac
- Should be performed in at least two planes
- 3D gradient-recalled echo imaging, a rapid technique, can also be used to image the heart
- Cardiac MRI is also performed to assess congenital disease
- Newer cine-2D GRE sequences can demonstrate flow patterns suggestive of valvular stenosis and regurgitations