2 IQ : Acquisition Time Objectives...Describe types of data acquisition modes....compute acquisition times for 2D and 3D scans.
3 2D Acquisitions The 2D mode acquires and reconstructs raw image data into two dimensional images whose brightness is proportional to the intensity of the MRI signal from the corresponding protons. This slice excitation method is followed by frequency and phase encoding to produce an image. Because the spatial encoding takes place in two dimensions the images are labeled 2D. Phase Encoding 2D slices Frequency Encoding
4 2D Multi-Planar Acquiring multiple images within a single acquisition or time frame means that the order of slice excitation is odd-numbered first, then even numbered images. All slices are acquired within the TR period Multi-Planar acquisition All 5 slices in the same TR
5 2D Sequential Acquiring an image sequentially means that all the excitation pulses ( # of phase steps x NEX x TR ) are delivered for one location before the process is repeated at another location. 1 Sequential acquisition Each slice is completed before moving on to the next slice
6 3D Acquisition In volume or 3D imaging, a wide RF pulse is delivered to excite an entire scan volume or slab. Spatial encoding must then be done in the phase, frequency and slice axes. Slice Encoding Phase Encoding 3D Volume Frequency Encoding
7 3D Acquisition 3D Slab
8 Progress Check 2D: TR x Phase Matrix x NEX = Acquisition Time 3D: TR x Phase Matrix x NEX x # of Slices = Acq. Time TR- TR is measured in milliseconds and is the time between RF pulses that repeat the pattern. Phase Matrix - The system collects data from the slice once for each phase encoding step. NEX- The number of times each set of phase encoding steps is repeated is called a NEX or Number of EXcitations. Calculate the scan times below using the appropriate formula
9 15. I.Q.: Acquisition Time 16. There are multiple acquisition modes: 2D: sequential and multi-planar 3D / volume MRS/ spectroscopy Cine 2D scan time formula: TR x phase x NEX 60,000 3D scan time formula: TR x phase x NEX x slices 60,000 2D & 3D are standard Go to QLB Pomp and ZIP
10 Module 9: IQ: Spatial Resolution
11 Spatial Resolution Objectives...Recall and explain the scan parameters that affect spatial resolution....compute spatial resolution given specific scan parameters....evaluate images acquired with varying spatial resolution.
12 Spatial Resolution How close two objects can be before they can be distinguished as separate objects.
13 Spatial Resolution Three parameters affect spatial resolution: FOV Matrix Slice Thickness Small FOV s, large matrices and small slice thickness result in high spatial resolution. #Phase Row FOV (cm) #Frequency Column Voxel Slice Thickness (mm)
14 FOV FOV determines the amount of anatomy displayed on the image. The larger the FOV, the larger the pixel size, and therefore the resolution is decreased. FOV can be changed in 1 cm intervals. Pixel size Pixel size 24 cm FOV 16 cm FOV
15 Progress Check Calculating Pixel Size, Area and Volume FOV / # phase steps = phase dimension FOV / # frequency steps = frequency dimension Phase x frequency = pixel area Pixel area x slice thickness = voxel volume Calculate the voxel volume for the image parameters listed below
16 Progress Check Slice Thickness change Image A: Thickness = 3 mm Image B: thickness = 5 mm Image C: Thickness = 10 mm SNR: Spatial resolution: Contrast: Time:
17 Progress Check FOV change Image A: FOV = 18 cm Image B: FOV = 22 cm Image C: FOV= 25 cm SNR: Spatial resolution: Contrast: Time:
18 Module 10: IQ: SNR
19 IQ: SNR Objectives View images for SNR and spatial resolution trade-offs....quantify (calculate) the change in SNR when a parameter has been changed.
20 Signal to Noise Ratio SNR is the ratio of the amplitude of the MR signal to the amplitude of the noise. Noise is the undesirable signal that is generated from the patient, the environment and the system electronics. signal noise
21 Signal to Noise Ratio SNR TIME TR NEX Resolution FOV Matrix Slice Thick Exception to rule : Receive Bandwidth
22 Receive Bandwidth Readout Window + 16 khz X gradient TR Readout Window + 32 khz X gradient TR
24 Receive Bandwidth Tradeoffs RBW SNR Chemical TE Motion Shift Artifact H 2 0 Chemical shift The frequency difference between protons bound in fat versus water Fat (+) (-) approx. 220 Hz ( 1.5T ) approx. 143 Hz ( 1.0 T ) approx. 74 Hz (.5T ) approx. 35 Hz (.2T )
25 I.Q.: Spatial Resolution/SNR 18. Voxel volume = I.Q. Tradeoffs FOV x FOV x slice phase freq SNR Isotropic voxels occur when all sides (height, width, depth) are equal. Time RBW is an exception Resolution
29 I.Q. : Image Contrast Objectives Identify scan timing parameters that determine image contrast....identify changes in image contrast and SNR when scan parameters change.
30 Contrast Weighting Images are labeled according to the internal factors predominately responsible for the variations in signal intensities The goal in MR is to select a pulse sequence and timing parameters that will cause one of the three contrast mechanisms to predominate over the other two.
31 Contrast Weighting T1-weighted images result when the variation in longitudinal regrowth creates transverse magnetization differences which predominate over other contrast mechanisms. T2-weighted images result when the rate of nuclear dephasing predominates over the other contrast mechanisms. Proton density-weighted images result when the number of nuclei in a tissue is predominately responsible for the image contrast.
37 T2* The sum of T2 and T2 effects that influence transverse decay. 1/T2 + 1/T2 = 1/T2* T2 is the transverse decay due to magnetic field inhomogeneities, chemical shift of the second kind and patient induced magnetic susceptibilities. T2 decay echo T2* decay
38 180 degree RF pulse Initial 90 0 pulse. Vectors decay and pulse applied. Fast catch up with slow components. Vectors are rephased.
39 Dephasing TE controls dephasing which controls T2 effects 37% 16% 5% 2% 0% S i g n a l csf fat brain-g brain-w 1 T2 2 T2 3 T2 4 T2 5 T2 Time (TE)
40 Spin Density Hydrogen content Mobility of hydrogen Field strength affects the number of hydrogen protons that are parallel with B O.
41 Tissue Contrast Relaxation recovery ( T1 ) The time it takes the net magnetization to return to B 0. Relaxation dephasing ( T2 ) The time it takes the protons to dephase and thus for the net vector to decay. Nuclear density ( PD ) The number of nuclei that comprise the vectors from various tissues.
42 Scan timing parameter chart TR (T1) TE (T2) Tissue Intensities for the brain T > white/gray = grayer CSF = brighter as TE > fat = dark/bright T : 1.5T : 1.0T : 0.5T : 0.2T 10-15ms minimum (fractional echo) white = light gray gray = gray CSF = dark fat = bright PD 2000 > minimum (fractional echo) gray = light gray white = gray CSF = dark fat = bright
49 TR Scan Parameter Trade-offs Class Activity SNR Sp. res. Time Cont. Cont. Cont. T1 PD T2 TE T1 PD T2 NEX Slice Thickness FOV Receive Bandwidth Contrast is directly affected by TR, TE, TI, and flip angle. SNR changes can enhance or obscure contrast but cannot change the image weighting from one type of contrast to another. Frequency Phase
50 I.Q.: Contrast Time (TR) T ms PD T1 T1 Saturation Min. Sat. Long TR Max Sat. Short TR T2 Dephase Min dephase Short TE Min. dephase Short TE T2 Time (TE) 300 ms T2 T2* Min sat. Long TR Min sat. Small flip Short TR ( ) Max dephase Long TE Short TE
51 Progress Check Slice Thickness change Image A: Thickness = 3 mm Image B: thickness = 5 mm Image C: Thickness = 10 mm SNR: Spatial resolution: Contrast: Time:
52 Progress Check FOV change Image A: FOV = 18 cm Image B: FOV = 22 cm Image C: FOV= 25 cm SNR: Spatial resolution: Contrast: Time:
Module : Spin Echo Spin Echo Objectives Review the SE PSD. Review the concepts of T, T, and T*. Spin Echo PSD RF Gz Gy 90 80 Gx Spin Echo - SE Spin echo is a standard pulse sequence on Signa MRi/LX and
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