Preparation for Scanning, Positioning the Phantom

Nuclear Associates 76-908 & 76-907

Operators Manual

2.2 Preparation for Scanning

2.2.1Positioning the Phantom

The 3DRAS phantom can be placed in a head coil or a body coil. The center of the phantom should coincide approximately with the center of the RF coil. The UAL phantom can be used only with a body coil.

2.2.2 Scanning Parameters

For all measurements, scan conditions should be carefully recorded. Scan conditions records should include:

•Any image processing which may have been used

•Field-of-view or zoom factor

•Image matrix size

•Imaging coil

•Number of signal (excitation) acquisitions

•Phantom and phantom material

•Pulse sequence name or code and software version number

•RF power settings

•Scan timing parameters (TE, TI, TR)

•Slice excitation order

•Slice number and thickness

•Tuning parameters

2.3 Tests with 3D Resolution and Slice (3DRAS) Phantom

2.3.1 Resonance Frequency

Resonance frequency is defined as that RF frequency (f) which matches the static B-field (Bo) according to the Larmor equation. For protons, the Larmor frequency is 42.58 MHz/Tesla, e.g., for a 1.5 Tesla system, the resonance frequency should be 63.87 MHz.

Prior to the performance of any imaging protocol, the resonance frequency must be checked first. Changes in the resonance frequency reflect changes in the static magnetic field (B-field). Changes in the B-field may be due to superconductor "run down" (typically on the order of 1 ppm/day, e.g., about 60 Hz/day at 1.5 Tesla), changes in current density due to thermal or mechanical effects, shim-coil changes, or effects due to external ferromagnetic materials.

The effects of off-resonance operation relate primarily to a reduction in image signal-to-noise. Secondary effects are reflected in image linearity due to the summation of the image gradients with the inconsistent static B-field value.

It is recommended that a resonance frequency check be performed prior to any measurements and each time a different phantom is used.

The phantom is positioned in the center of the magnet (with all gradient fields turned off) and the RF frequency is adjusted by controlling the RF synthesizer center frequency to achieve maximum signal. Some resistive systems may also allow adjustment of the magnet current in order to alter the magnetic field strength so as to achieve resonance. Most vendors will provide a specific user protocol for

2-2

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