Chapter 6 physics
Is k-space the image?
-Kspace is not an image
List key factors on how pulse sequences fill k space
1. RF excitation pulse centers the system in the middle of K space 2. Amplitude of phase encoding gradient determines how far up and down a line is filled. Steepness determines outer line positive and negative determines the size of the pixel in the phase direction 3. Polarity of phase encoding gradient determines whether a line in the top or bottom is filled. Positive top, negative bottom. 4. The amplitude of the frequency encoding gradient determines how far right and left or left and right of K space is filled, this determines the size of the pixel in the frequency direction . 5. Polarity of the frequency encoding gradient determines whether K space is filled left to right or right to left.
Define K-space
An area in the array processor where data on spatial frequencies are stored.
Discuss how K-space is filled from steep and shallow phase encoding gradients.
Central lines of K space are filled using shallow phase encoding slopes and the outer lines are filled using steep positive and negative phase encoding slopes.
Discuss the characteristics of 3D Imaging
Data are acquired from an entire volume of tissue rather than in separate slices. Slices can be contiguous and not gap.
Define partial averaging:
Filling only a portion of K space with data and putting zeros in the remainder
Discuss the characteristics 2D Imaging
Fills one line of K space for slice 1 and then goes on to fill the same line of K space for slice 2, etc.
Identify what factor determine number of data points to be collected
Frequency matrix
Define conjugate symmetry as it relates to k-space:
Identical to top and bottom, left and right. It is used to reduce scan times in some imaging options.
List factors that determine pixel size:
Image matrix: determined by the number of data points in K space. FOV: determined by the difference between each data point in K space in either direction. It is inversely proportonal to the size of the FOV in that direction.
Define Fast Fourier transformation:
Mathematical conversion of frequency/time domain to frequency/amplitude.
Define number of excitations (NEX), and give examples of additional acronyms.
Number of times a line of K space is filled with data. (NSA, NEX ) The signal can be sampled more then once by maintaining the same slope of the phase gradient over several TR's
Define partial echo:
Performed when only part of the echo is read during application of the frequency encoding gradient and extrapolating the remainder in K space.
Identify what factor determines the number of lines in K-space
Phase matrix
Define sampling rate
Rate at which the frequencies are sampled or digitized during the acquisition window, number of data points per second.
Describe different ways K space is filled to reduce scan times: Spiral
Read out and phase gradient switch their polarity rapidly and begins at the center of K space and goes out in a spiral pattern.
Define receive bandwidth
Rebate of frequencies that are sampled during readout. (the range of frequencies of the signal that is read and digitized and the SPEED) Range and speed.
Define frequency resolution:
Refers or the resolution of spatial frequenies in K space
Define spatial resolution
Refers to resolution in the image. The ability to distinguish two points in the image as separate.
Describe different ways K space is filled to reduce scan times: Propeller
SNR and CNR is increased because the central portion is acquired every TR and that's where signal and contrast is. Also reduction in motion artifact due to motion averaging.
3D volume scan time formula
ST=PMxTRx#ofslicesxNEX
Fast Spin Echo (FSE) Scan Time Formula
ST=PMxTRxNEX/ETL
State the scan time formula.
ST=TRxPMxNEX
State the artifact that occurs when the Nyquist theorem is not followed.
Sampling at less than once per cycle represents a completely incorrect frequency that leads to an artifact called ALIASING (wrap around).
State the Nyquist theorem
States that a frequency must be sampled at least twice in order to reproduce it reliably
State how the phase gradient selects each line of K-space
Steep slope phase encoding gradient select both positive and negative outer lines with low signal amplitudes, where as the shallow phase encoding gradient select the central lines with high signal amplitude.
Define single-shot (SS) imaging
Techinque that fills K space in one shot. (One single echo train)
Define parallel imaging:
Technique that uses multiple coils to fill segments of K space.
Identify which parameters control the scan time
The TR, Phase Matrix and NEX
Each pixel is given color on a gray scale corresponding to
The amplitude of the frequency
Identify which areas of K-space have a low signal amplitude
The center portion has high signal amplitude and low spatial resolution
And what areas determine Image contrast-
The central line portion due to its low resolution
Define sampling time
The duration of the frequency or Read out Also known as acquisition window.
Describe how lines of k-space are numbered
The lines are numbered with the lowest number near to the cetral axis and the highest number towards the outer edges. The top half lines are positive and the bottom half is negative. And it determines the polarity of the phase encoding gradient.
And what areas determine Image resolution
The outer line portion due to its high resolution
Identify which areas of K-space have low signal amplitude
The outer portion has low signal amplitude and high spatial resolution
Identify which gradient changes amplitude (slope) from one TR period to the next.
The phase encoding gradient
Describe frequency matrix
the amount of pixels along the frequency axis of the image And the same number of data points of each row of K space
Describe the phase matrix
the amount of pixels along the phase axis of the image. And the same number of data points of each column of K pace
Frequency encoding gradient is also known as
the read out or measurement gradient
