1 Weber State University Radiologic Technology 4603 MRI Physics and Instrumentation Instructor: Rex T. Christensen MHA R.T. (R) (MR) (CT) (ARRT) CIIP _____________________________________________ Contact Info: E-mail: [email protected]Phone: 435.770.2643 (cell) 801.626.8122 (office) Address: Radiology Sciences Weber State University 3925 University Circle Ogden, UT 84408-3925
18
Embed
Weber State University Radiologic Technology 4603radpacs.weber.edu/Images/R_Christensen/MRI/4603/PowerPoints/RT4… · Weber State University Radiologic Technology 4603 ... MRI vs.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1
Weber State University
Radiologic Technology 4603
MRI Physics and Instrumentation
Instructor: Rex T. Christensen MHA R.T. (R) (MR) (CT) (ARRT) CIIP
Proper coil placement is vital to getting good images.
9
Phased Array Coils:
Multiple surface coils.
High SNR low sensitivity area (single)
Shim Coil:
This coil enhances the magnetic field
homogeneity.
Homogeneity = Magnetic field uniformity.
Measured in parts per million (PPM)
10
Shim Coil:
Two kinds of shim coils:
• Passive (physical way to shim)
• Active (electronic way to shim)
Types of Magnets:
• Permanent
• Resistive
• Superconducting
Measured in Tesla (T)
1 Tesla = 10,000 Gauss(Gauss is another form of measurement)
11
Permanent Magnets:
Resistive Magnets:
12
Superconducting Magnets:
Superconducting Magnets:
• Most common magnet.
• Only one with Cryogens. Why?
It allows conduction of electricity with no resistance.
• Most expensive.
13
Shielding a Magnet Room
Items to consider:
• Magnetic field strength
• Location
• Safety
• Cost
Shielding a Magnet Room
Types of shielding:
• RF shielding (copper)
• Magnet shielding
Passive (steel)
Active (reverse magnetic field)
14
Gradient coils are smaller (secondary)
magnetic fields that:
1. Select imaging planes.
2. Localize data spatially.
Gradient Coil: (secondary magnets)
Gradient Coil: (secondary magnets)
Z, X, Y Gradients:
• Z gradient coils are circular in shape. The coils are on opposite ends of the gantry.
Transverse slices are acquired by this gradient.
• X gradient coils are 4 saddle shaped coils in sets of two (located on the side of the gradient coil).
Saggital slices are acquired by this gradient.
• Y gradient coils are the same as the X gradient coils (located on the top and bottom of the gradient coil).
Coronal slices are acquired by this gradient.
15
16
Imaging Planes
Gradients
17
Gradient Amplitude and Rise Time
• Gradient Amplitude: Strength of the gradients. Measured in milli-tesla per meter (mT/m).
• Gradient Rise Time: The time it takes for the gradients to reach their maximum strength or amplitude. Measured in microseconds (ms).
• Slew Rate: The strength of the gradient over distance. Measured in milli-tesla per meter per second (mT/m/s).
Gradient Amplitude
Rise Time = Slew Rate
• Duty Cycle: The percentage of time per TR that the gradient is permitted to be at maximum amplitude, or the work time. Measured in percentage i.e. 50%.