Dual Drive Theory

The purpose of the dual drive feature is to improve image homogeneity.

Dual drive technology reduces shading caused by dielectric affect when scanning areas of the body that are more elliptical or irregular in shape (rather than circular), such as the thigh, pelvis, abdomen, and breast. The dual drive controls the amplitude and phase relationships between the two separate drive points on the system body coil and minimizes the dielectric effect. The dual drive system establishes an elliptical polarized RF field primarily in a horizontal direction.

Dual drive applies only to body transmit. Two identical body transmit paths are maintained from the exciter output to the body coil input. Dual drive only affects body mode imaging and receive-only surface coil (single channel and phased array) imaging.

A 3.0T system with dual drive amplifier and exciter drives the body coil in either Quadrature mode or Elliptical mode.

Elliptical mode is available as a customer-purchased option. Quadrature mode is always available.

Two types of Elliptical mode are selectable in the interface:

• Preset mode: A fixed set of values for attenuation and phase; the default mode for the end user in most scans. The presets vary based on the coil being used for a scan.
• Optimized mode: The operator sets the values of attenuation and phase on a per-patient and per-anatomy basis. Using Optimized mode is complicated and time consuming. Most operators do not use this option.

Most system tools and service protocols are run only in Quadrature mode. One exception is Uniformity, which lets the user choose Quadrature or Preset mode.

Anatomies with circular shapes, such as head, knees, and wrists, are not improved through the use of dual drive technology. Therefore, dual drive is not applicable to any local TR coils (TR split head, TR knee, TR wrist, etc.).

The dual drive feature includes system tools for testing and calibration.

The block diagrams show configurations for the equipment room and the scan room.

The illustration below shows a block diagram of the dual drive RF amplifier.

Most of the hardware components in the transmit chain are different between a dual drive system and a single drive system. However, in the receive chain, only the body receive quadrature preamp/combiner module is different.

3.0T systems with dual drive have no remotely located dummy load (in the hybrid or magnet room) to absorb reflected power. Instead, all reflected power from the RF body coil is absorbed by use of internal circulators and dummy loads inside the RF amplifier. Therefore, during normal operation, it is not unusual for large values of reflected power in excess of 4:1 VSWR to be present on the channel 1 and channel 2 body RF transmission lines. The 3.0T DTX1 exciter drives the channel 1 and channel 2 RF amplifier gain stages with two RF outputs, each of which can be independently set with respect to amplitude and phase. However, both outputs use the same exciter unblank control signal.

In dual drive, the naming convention of CH1 for channel 1 and CH2 for channel 2 is used. CH2 exits the exciter at J13 and leads with respect to CH1 (at J12). CH2 is the adjustable channel for elliptical mode. The dual drive RF amplifier has two independent RF inputs and uses one unblank signal to control dual body output ports. Each body output port is capable of 15 kW PEP at 1 kW average.

During body transmit, the RF amplifier receives two inputs from the exciter (J14 and J21), and there are two outputs on J4 and J22. During head transmit, the RF amplifier receives one input from the exciter (J14), and there is one output on J3.

The driver module provides a single body TR bias signal to the dual drive RF amplifier, as in single drive systems. The amplifier couples this single bias signal onto the two body transmit output ports. Therefore, both body channel transmit cables carry both RF and common body TR DC signal (as in previous systems). What is different is that an open or short-circuit failure on either CH1 or CH2 transmission line can cause a body TR bias fault. Locating the source of a fault involves using the TR DD System Path Check diagnostic and comparing the reported body TR bias current values while disconnecting the CH1 and CH2 transmission lines one at a time.