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MRI formulas

QuestionAnswer
TR X Phase Matrix X NEX Spin Echo
TR X Phase Matrix X NEX X ETL Fast Spin Echo
TR X Phase Matrix X NEX Gradient Echo (assuming 2D)
TR X Phase Matrix X NEX X # of slices Gradient Echo 3D acquisition
FOV➗ matrix Pixel size
FOV ➗ Matrix (phase and frequency) ➕ slice thickness measurement Voxel size
Pixel area X slice thickness Voxel volume
TR x Phase Matrix x NSA/NEX = Scan Time in a Spin Echo sequence, Divide by 1000 to convert ms (milliseconds) into seconds Scan time: Spin Echo
Fast Spin Echo Sequence : TR x Phase Matrix x NEX ÷ ETL, ÷ 1000 to convert to seconds Scan time: Fast Spin Echo
3D sequence scan time : TR x Phase Matrix x NSA/NEX x # Slices = 3D Scan Time, ÷ 1000 Scan time: 3D Gradient Echo
FOV ÷ phase matrix = phase dimension pixel FOV ÷ frequency matrix = frequency pixel Pixel Size Dimensions (phase and frequency)
FOV ÷ Matrix = pixel size Pixel Size
Multiply phase pixel size x frequency pixel size = Pixel area (answer squared 2) Pixel Area
Pixel area x slice thickness = Voxel volume (answer cubic 3) Voxel volume
SNR increase calculated by the square root (√) of the increase NSA/NEX and SNR
Decreasing BW by factor of 2 increases SNR by √2 Bandwidth and SNR
Reducing the FOV decreases SNR by a factor of the FOV2. Example: Reducing the FOV from 40cm to 20cm results in SNR reduced to 1⁄4 the original SNR, due to the voxel volume being reduced by a factor of 4 FOV and SNR
60,000 ÷ heart rate (60 seconds per minute, 1000 milliseconds per second) Calculate effective TR
Effective TR (see above) – (delay and trigger window). For example, Heart Rate 80bpm, Trigger delay 50ms, Trigger window 10%, what is the available imaging time? 60000 ÷ 80 = 750ms. 750 – 125 (75{10% of 750} + 50{delay}=125) = 625 ms total Imaging time Calculate available imaging time in a Cardiac MR Rate
Created by: zach1710
 

 



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