8: Data Manipulation Flashcards
Slice Select Gradient (SSG)
produces a linear shift of the magnetic field resulting in a positive and negative tilt.
Slice Select Gradient Amplitude
The strength of the energy used to create the variation
Increasing Slice Select Gradient Amplitude will result in:
a steeper tilt allowing the ability to scan thinner slices.
After the SSG selects the slice:
fill in frequency and phase information (encoding) with the echoes gathered from the pulse sequence
Raw data
K space image, no anatomy
Center - Contrast
Peripheral - Resolution
K Space
raw data storage, collection of a variety of numerical values corresponding to the spatial frequency and phase values of the resultant pulse.
Data is scattered and not identical to an MR image.
K Space Center
Contrast
K Space Peripheral (boundry)
Resolution, Frequency, Phase, Matrix
What is the most essential contributor to image contrast?
effective TE
Where is TE located in K Space?
Closest to the center
K Space filling
occurs when high and low spatial frequencies fill empty k space.
Where are lower spatial frequencies found in K space?
Middle
Where are higher spatial frequencies located in K space?
Boundaries, Peripheral
The lines of K Space filled during each TR period is (directly or indirectly) related to the number of echoes collected per TR.
Directly
What is found in the center of K Space? (3)
- High amplitude
- Lower spatial frequency
- Contrast
What is found in the peripheral of K Space? (3)
- Low amplitude
- Higher spatial frequency
- Resolution
3 Types of K Space Filling:
- Cartesian
- Spiral
- Keyhole
Cartesian K Space filling:
traditional, Fills data row by row from left to right or left to right
Spiral K Space filling:
beginning in the center, raw data is filled like a backwards maze beginning with contrast data
Keyhole K Space filling:
Normally used for dynamic studies, in which lower spatial frequency data (contrast) is filled to show infiltration of contrast - higher spatial frequency is filled once. Repetitively fills the contrast over time, gets brighter.
Pituitary, Prostate
Spiral K Space filling is also called:
Elliptical filling
MaxIP (Maximum Intensity Projection)
Post processing technique which allows:
1. Cutting of subtracted (dark) background
2. Highlighting of high intensity blood vessels
3. Manipulation of images to visualize blood vessels in various projections and positions
MinIP (Minimum Intensity Projection)
MRA or CT Post-processing technique which allows for cutting of hyperintense or bright structures.
Ideal application for hypodense structures such as the lungs, biliary tree, and pancreatic duct.
MPR (Multiplanar Reconstruction)
3D post-processing technique which allows for reformatting of in both slice and acquisition planes.
MPR is only possible if the image is a _______ due to: (3)
3D Isotropic volume due to
1. High SNR
2. Thin slices
3. No gaps
unlike 2D.
Isotropic:
Volumetric ROI that has EQUAL width, height, and depth of each voxels
Rubix cube
Anisotropic
Volumetric ROI that has UNEVEN width, height, and depth of each voxels
During 3D acquisitions, voxels can be configured to be: (2)
- Rectangular - linear anatomical structures
- Square - routine anatomy
To maintain isotropic volume, 4 categorizes need to be accounted for:
- volume thickness (mm)
- number of slices
- FOV (mm)
- Matrix (phase only)
Formula for maintaining Isotropic Volume:
Slice Thickness FOV
___________________ = ________________
Number of slices Matrix (phase)
If a technologist selects a 3D volume slab with a FOV of 20cm, a matrix of 512 x 288 and a volume with a 24mm thickness. How many slices are needed to maintain an isotropic 3D sequence?
Slice Thickness FOV
___________________ = ________________
Number of slices Matrix (phase)
24mm 200mm
__________ = _________________
solve for x 288 phase lines
cross multiply
200x = 24 x 288
200x = 6,912
6,912 / 200 = 34.56 slices
Answer = 35 slices
Subtraction
technique where pre-contrast T1 is digitally subtracted from identical post-contrast T1 images.
Apparent Diffusion Coefficient (ADC)?How is it measured?
Diagnostic assessment tool used to evaluate diffusion of water within a specific area of tissue by mapping acquisitions with different b-values. mm2/s (millimeters squared per second)
Diffusion coefficients:
a set of true numerical values for certain diffusivity rates
