MR IMAGE QUALITY PARAMETERS.pptx
- 1. FACTORS AFFECTING MR IMAGE QUALITY
- 2. FACTORS INFLUENCING MR IMAGE QUALITY • Signal to noise ratio (SNR) • Contrast to noise ratio (CNR) • Spatial resolution • Scan time
- 3. Signal to noise ratio (SNR) • Ratio of amplitude of signal received to the average amplitude of noise
- 4. Signal to noise ratio (SNR) Depends on • Proton density • Voxel volume • TR,TE and FA • Receive band width • Coil type
- 5. How to improve SNR? • Use SE pulse sequences • Try to avoid short TR and long TE • Use correct coil and ensure proper tuning • Use coarse matrix • Use large FOV • Select thick slices • Use more nex
- 6. Parameter Advantages Disadvantages TR increased Increased SNR Increased number of slices per acquisition Increased scan time Decreased T1 weighing TR decreased Decreased scan time Increased T1 weighing Decreased SNR Decreased number of slices per acquisition TR: Is the time we wait between successive pulse sequences applied to the same slice.
- 7. TR = 4500 ms Number of slices =15 TR = 3500 ms Number of slices =11 Increased TR = Increased SNR
- 8. Increased TR = Increased PD contrast, Signal and Scan time TR 500 ms, TE 10 Scan time :2min 48 sec TR 1000 ms, TE 10 Scan time:5min 48 sec
- 9. Parameter Advantages Disadvantages TE increased Increased T2 weighing Decreased SNR TE decreased Increased SNR Decreased T2 weighing TE: It is the time from the original RF pulse to the peak of the echo
- 10. TE = 156 ms TE = 83 ms Increased TE = Decreased SNR
- 11. Parameter Advantages Disadvantages Slice thickness increased Increased SNR in all tissues Increased coverage of anatomy Decreased spatial resolution and partial voluming in slice select direction Slice thickness decreased Increased spatial resolution and reduced partial voluming in slice select direction Decreased SNR in all tissues Decreased coverage of anatomy Slice Thickness: It is the depth of a three dimensional volume of imaged area
- 12. SL = 10 mm SL = 3 mm Increased Slice thickness = Increased SNR
- 13. Parameter Advantages Disadvantages NEX increased Increased SNR in all tissues Reduced flow artifact due to signal averaging Direct proportional increase in scan time NEX decreased Direct proportional decrease in scan time Decreased SNR in all tissues Increased flow artifact due to less signal averaging No. of Excitations (NEX) / No. of Signal Averages (NSA): No. of times we collect data per phase encode step and average the information to produce one image.
- 14. NEX = 5 Scan Time =1.46 sec SNR=100% NEX = 3 Scan Time =1.06 sec SNR= 77% Increased NEX = Increased SNR, Scan time
- 15. Parameter Advantages Disadvantages Matrix increased Increased spatial resolution Decreased SNR if pixel size decreases. If pixel size remains same SNR will increase because more encodings are performed Increased scan time Matrix decreased Increased SNR in all tissues if pixel size increases. If pixel size remains same SNR decreases as fewer phase encodings are performed Decreased scan time Decreased resolution Matrix: No. of frequency encodings x No. of phase encodings.
- 16. PIXEL & VOXEL
- 18. MATRIX=512x512 MATRIX=256X256 Increased matrix = Increased Spatial resolution
- 19. 256 x 160 SNR = 98% Time=2 min 48sec 256 x 256 SNR = 77% Time=4 min 24sec 192 x 160 SNR = 118% Time=2 min 48sec Increased matrix = Increased Spatial resolution
- 20. Parameter Advantages Disadvantages FOV increased Increased SNR Increased SNR in all tissues Decreased likelihood of aliasing Decreased spatial resolution FOV decreased Decreased SNR in all tissues Decreased coverage of anatomy Increased likelihood of aliasing Increased spatial resolution Field of view (FOV): It is the total dimension of the anatomic region of interest and is defined by the Matrix & resolution in two dimensions.
- 21. FOV = 24 cm FOV = 12 cm Increased FOV = Increased SNR and decreased spatial resolution
- 22. FOV = 12 cm SNR=25% FOV = 24 cm SNR=98%
- 23. Parameter Advantages Disadvantages ETL increased Decreased scan time Increased contrast mix Decreases no. of slices May increase blurring artifact ETL decreased Increased scan time Increased scan time Increases no. of slices Echo Train Length (ETL): Total number of echoes collected during one repetition (TR) of one slice sampled using varying phase encoding steps for each echo (or) number of 180 RF pulses used per TR. Scan time is inversely proportional to ETL.
- 24. ETL= 15 Scan time= 1 min 44 sec Slices=20 ETL = 5 Scan time=4 min4 sec Slices = 46 Increased ETL = Decreased scan time
- 25. Parameter Advantages Disadvantages Flip Angle Increased More SNR -- Flip Angle decreased -- Less SNR Flip Angle (FA): Control the amount of vector component that will be forced by RF into transverse plane.
- 26. FA = 90 ° FA =15° SE GRE More FA= Better SNR
- 27. Parameter Advantages Disadvantages Receive bandwidth increased Decreased SNR Decrease of minimum TE Decreased in chemical shift Receive bandwidth decreased Increased SNR Increase of minimum TE Increased in chemical shift Receive Bandwidth: The range of frequency signals received from the object
- 28. BW = 20.83 BW = 7.81 Larger BW = Less SNR
- 29. Parameter Advantages Disadvantages TI increased Improved SNR and CNR Less no. of slices available per TR TI decreased More no. of slices available per TR No fluid saturation TI (or) Time from Inversion: It is the time between the inverting 180° RF pulse and the 90° degree excitation pulse .
- 30. TI = 2200 ms TI = 150 ms Increased TI = Increased SNR, CNR and decreased number of slices
- 31. 1.5 mm spacing 0 mm spacing Spacing vs SNR Minimum 30% spacing necessary to avoid cross excitation
- 32. Contrast to noise ratio (CNR) • Difference in the SNR between two adjacent areas • Depends on same factors like SNR • CNR can be improved by contrast medium and MTC
- 33. T1W T1W+ C MTC MTC improves contrast resolution
- 34. Spatial resolution • Ability to distinguish between two points as separate and distinct • Depends on slice thickness, FOV and matrix
- 35. How to improve Spatial resolution? • Selecting thinner slices • Selecting fine matrix • Selecting small FOV • Selecting rectangular FOV (where ever possible)
- 36. SL = 10 mm SL = 3 mm Smaller slice thickness = Better spatial resolution
- 37. FOV = 12 cm FOV = 24 cm Smaller FOV = Better spatial resolution
- 38. 256 x 256 SNR = 77% Time=4 min 24sec 192 x 160 SNR = 118% Time=2 min 48sec Increased matrix = Increased Spatial resolution
- 39. Scan time • Depends on TR x N x NEX (NSA) • Better to reduce scan time without compromising diagnostic image quality
- 41. T1W T2W PD DWI GRE FLAIR Difference sequences with different image quality