Interventional Cardiovascular Magnetic Resonance

Communication, Monitoring and Image Display

Rapid CMR requires rapid gradient switching that causes substantial acoustic noise. This noise prevents verbal communications between operators, the patient, and staff. Wireless noise-cancelling headsets are used at NHLBI ( Fig. 47.2 ). This versatile system allows staff and patients to speak simultaneously on open microphones and allows separate and parallel conversations with patients and among staff.

Gradients, RF interference, and magnetohydrodynamic effects severely distort the electrocardiogram (ECG). Electronic filters enable heart rate monitoring; however, ECG waveform morphology is not readily interpretable for ST-segment monitoring of myocardial ischemia or injury. Multichannel invasive pressure waveform monitoring, temperature monitoring, and oxygen saturation monitoring can use commercial systems, without modification from x-ray catheterization laboratories. Several manufacturers are developing dedicated hardware that will interface with existing catheterization laboratory high-fidelity hemodynamic recording systems. In the meantime, open source kits for local assembly of suitable hemodynamics interfaces are available ( http://nhlbi-mr.github.io/PRiME ).

Interventional catheterization suites require continuous operator monitoring of multiple information streams, including hemodynamics, imaging control, and interventional images. This is similarly true during iCMR procedures, especially using active devices. MRI-conditional liquid crystal display (LCD) video displays are available from most scanner manufacturers. We prefer two banks of less expensive LCD video projectors that project onto a fabric screen, which depict hemodynamics, scanner interface, and 3D-rendered images separately, one at each end of the CMR system ( Fig. 47.2 ). The projectors are housed in RF-shielded boxes and video signals are passed into the room via fiber optic cables through the waveguide.

Interventional Cardiovascular Magnetic Resonance Real-Time Scanner Interface

The iCMR scanner user interface requires several key features not typical for diagnostic CMR. Commercial or investigational graphic user interface based scanning host computer systems are available from most systems vendors (Interactive Front End, Siemens ; MR Echo, General Electric; eXTernal Control, Philips Healthcare) and as standalone commercial (RT Hawk, Heart Vista) or open-source solutions (Vurtigo, Sunnybrook Health Sciences Center, http://www.vurtigo.ca ).

One of the most useful features is the colorized display of individual receiver channels that are attached to “active” catheter receiver coil devices. This makes catheter devices readily apparent and has proven useful in vivo. Another is interactive “projection-mode” imaging wherein slice select is disabled and catheter devices are displayed as the operator is used to seeing them under conventional x-ray fluoroscopy. For tracking balloon catheters filled with dilute gadolinium (Gd) contrast during CMR catheterization, we find it helpful to interactively adjust slice thickness and saturation magnetization preparation, to help find the catheter tip should it move outside of the selected plane, and to interactively adjust temporal resolution for catheterization steps requiring fine motor control. Operationally, we find it useful to display multiple slices during interventional procedures, both separately and combined in a 3D-rendered image ( Fig. 47.3 ). Other laboratories are experimenting with voice-control, automatic adjustment of slice location to correspond to catheter position, and automatic adaptation of imaging parameters to speed of catheter manipulation.

Safety Considerations

Patient care staff must undergo formal comprehensive training in CMR safety. For example, the hospital emergency medical response team is unlikely to be familiar with CMR operations and may unwittingly jeopardize themselves, their colleagues, or the patient by rushing into the room with a steel oxygen tank. Rapid patient evacuation from the CMR catheterization laboratory needs to be practiced repeatedly in mock drills. Ferromagnetic objects that cannot be removed from the room must be attached to the wall during CMR guided procedures. It is good practice for all devices on wheels in the x-ray section of the iCMR suite (e.g., anesthetic machine, intraaortic balloon pump, or defibrillator) to be tethered to the wall or floor. Oxygen, inhalational anesthetic, and vacuum for suction can be fed through RF-protective wall ports, eliminating in-room cylinders. At NHLBI, we drill emergency evacuation from the CMR room on a quarterly basis. Swift evacuation relies on clear delineation of roles for all staff. In an emergency, we can start chest compressions within 10 seconds and move the patient back into the x-ray room for defibrillation in under 1 minute. In the near future, CMR-conditional external defibrillators should be commercially available so that arrhythmias can be managed without evacuating the patient from the scanner.

RF energy from CMR transmit coils will concentrate on long conductive devices causing local heating, potentially leading to burns. Box 47.1 lists factors associated with heating, especially long conductive devices and cables. This complex problem is described later under “active” catheter devices. Interventionists must also take care that connections to intravascular coils as well as surface coils do not inadvertently form loops, which can cause patient burns.