MR Essentials - Image Quality - USA (copy)

Welcome to the MR Essentials - Image Quality Online Training. Image quality is the product of complex interactions between the spatial resolution of the structures acquired in the image, signal strength, and tissue contrasts obtained in relationship to unavoidable noise. The following topics will be covered in this course: Signal-to-Noise Ratio Spatial Resolution Contrast-to-Noise Ratio Scan Time      Upon successful completion of this course, you will be able to: Identify the imaging parameters that affect signal-to-noise ratio Explain how spatial resolution is influenced by specific imaging parameters Describe different techniques that are applied to help reduce scan time Recognize the imaging parameters that affect image contrast Congratulations. You have completed the MR Essentials - Image Quality Online Training course. Listed below are the key points that you have learned about factors that influence MR Image Quality. Take time to review the material before you proceed to the final quiz. Identify the imaging parameters that affect signal-to-noise ratio Proton Density Voxel Volume TR, TE, and Flip Angle Number of Averages Receiver Bandwidth Coil Type Field of View Oversampling Field Strength  Explain imaging parameters that influence spatial resolution Field of View Pixel Size Matrix Slice Thickness Describe the different techniques to help reduce scan time Reduce the Measurement Matrix When a smaller measurement matrix with a lower phase resolution is selected, the number of phase-encoding steps is fewer. The number of phase-encoding step is directly proportional to the measurement time.   Reduce the Field of View (FoV) If you cut the field of view in half in the phase encoding direction, half as many phase-encoding steps are required. The measurement time is directly proportional to the number of phase encoding steps.   Half-Fourier Technique Only half of the raw data matrix is filled with data in the phase-encoding direction.   Phase Partial Fourier Technique Works the same way as the half-Fourier technique, only part of the k-space is filled in the phase-encoding direction.   Integrated Parallel Acquisition Techniques (iPAT) Employs a sophisticated computational technique or reconstruction algorithm where each coil independently and simultaneously images a given volume. Recognize imaging parameters that affect imaging contrast Contrast resolution determines how well a specific technique differentiates between two tissues. Image contrast depends on the following: TR, TE, flip angle Inversion Time Turbo Spin Echo T1 & T2 relaxation time Proton density Select this link to download the Course Review. Spatial Resolution - ability to distinguish between two points as separate and distinct, and it is controlled by voxel size. Voxel Size is affected by: Field-of-view Number of pixels or matrix size Slice Thickness  Field-of-view defined by field-of-view readout and field-of-view phase Matrix consists of rows and columns Pixel refers to individual picture element Voxel  defined in the slice plane by field-of-view and matrix and in the slice direction by slice thickness Pixel Size =  FoV/Matrix Size The smaller the FoV at a fixed matrix size, the higher the in-plane resolution The number of pixels per in-plane unit increases while the pixels decrease in size Smaller pixels create improved in-plane resolution      Matrix size determines resolution and measurement time. Measurement Time  = Phase Resolution x TR x Number of Averages Enlarging the measurement matrix will increase spatial resolution, as long as no other parameters change.     Matrix Relative Signal 128 4.0 256 1.0 512 0.25     As the slice thickness is increased, there is a decrease in spatial resolution. Thick slices reduce the resolution along the slice direction and produce an effect called partial volume averaging.      The contrast-to-noise ratio (CNR) in an MR image is the difference between the signal-to-noise ratios of 2 tissue types, (A and B). CNR is the effective contrast CNR = SNRA - SNRB = Difference in signal   SA                                             Difference in signal SB                                                                        A        B   SA                                                  Difference in signal SB                                                                                                    A        B   Factors that affect scan times include: TR (repetition time) Number of phase encodings Number of averages   The following adjustments can reduce scan time: Reduce the Measurement Matrix Reduce the Field of View (FoV) Half-Fourier Technique Phase Partial Fourier Technique Integrated Parallel Acquisition Techniques (iPAT)   Reducing Scan Times Learn about different ways to reduce scan times. Tab TitleTextReduce Measurement Matrix   Reduce Field of View   Half Fourier   Phase Partial Fourier   iPAT   Integrated Parallel Acquisition Techniques (iPAT) Scan time reduction technique Employs reconstruction algorithms and arrays of coils Each coil independently and simultaneously images a given volume Uses spatial information inherent in local array or matrix coils Noise in an image appears as a grainy random pattern. Noise is generated throughout the human body through Brownian motion of molecules. Signal-to-Noise Ratio (SNR) is the ratio of the amplitude of signal received to the average standard deviation of the noise. The following parameters affect SNR: Proton density Slice Thickness Voxel volume TR, TE, and flip angle Averages Receiver Bandwidth Coils Field of View (FOV) Oversampling Field Strength        Factors Affecting SNR Learn about factors that affect SNR. Checklist TitleChecklist TypeChecklist ContentProton DensityHTML If the voxel size is increased by measuring a thicker slice, the signal intensity increases because there are more hydrogen nuclei contributing to the signal strength.   Slice ThicknessHTML SNR is directly proportional with slice thickness.   Voxel VolumeHTML Voxel Size is affected by: Field-of-View (FoV) Number of pixels or matrix size Slice thickness Pixel Area = Field-of-View                       Matrix Size Larger Voxels have a higher SNR.   TR, TE, Flip AngleHTML Parameters that affect image contrast & SNR: TR or repetition time Controls the contrast between tissues in an image due to T1 relaxation A long TR increases SNR but reduces T1 contrast TE or echo time The time from the initial RF excitation to the center of the MR Echo formation Long TE reduces SNR but increases T2 contrast Flip Angle Controls the amount of transverse magnetization that is created and then induces a signal in the coil   AveragesHTML Increasing the number of averages will increase the SNR.   Receiver BandwidthHTML Reducing the bandwidth: Increases the SNR Increases the minimum TE Increases chemical shift artifact   CoilsHTML Size of the receiver coil should be chosen to fit the volume of tissue being imaged.   Field of View (FoV)HTML Field of View (FoV) - square or rectangular image area to be measured (in mm) FoV determines what you can see in the MR Image Larger FoV increases SNR   OversamplingHTML Increase Oversampling Increase SNR Increases scan time Minimizes aliasing artifacts   Field StrengthHTML SNR increases linearly as Field Strength increases. SNR is proportional to B02/B0   Increase Matrix   Increases Increases Decreases Increase FoV   ------   Decreases Increases Increase Slice Thickness   ------   Decreases Increases Measurement Time Resolution Size