EXAM #14 — PHYSICS UNIT 07 Flashcards
Define: a. gradient magnetic field.
magnetic fields that change linearly over the magnet bore
Define: b. logical gradients.
slice select, frequency encoding, and phase encoding gradients
Define: c. spatial localization.
the process of locating each voxel of tissue in an imaging volume, and positioning it correctly in the image
Define: d. gradient slope.
the amount of change in strength of a gradient magnetic field
Define: e. magnet isocenter.
the center of the primary magnetic field in all three planes
Define: f. slice selection gradient.
when is it switched on?
the logical gradient that localizes tissue in the imaging plane, is switched on during RF transmission
Define: g. frequency encoding gradient.
when is it switched on?
logical gradient that localizes tissue perpendicular to the imaging plane- is switched on during signal reception
Define: h. readout gradient.
same as frequency encoding gradient
Define: 1. phase encoding gradient.
when is it switched on?
logical gradient that localizes tissue perpendicular to the imaging plane- is switched on before the refocusing pulse
Define: j. RF transmit bandwidth.
the range of frequencies of transmitted RF
Define: k. RF receive bandwidth.
the range of frequencies of received RF
Define: 1. slice thickness.
the amount of tissue that is imaged with each slice, affects spatial resolution
Define: m. sampling rate.
the frequency at which a waveform is sampled, or how many times per second the MR signal is sampled
Define: n. sampling time.
the amount of time during which a waveform is sampled
Define: o. k-space.
the frequency- and phase- encoded raw data map of an MR image
Define: p. image matrix.
the number of frequency and phase encoding steps used to spatially localize an image
Define: r. partial or fractional echo imaging.
a method of imaging whereby only half of k space in the frequency encoding axis is filled during acquisition, and the other half is calculated by the MR system
Define: s. partial or fractional averaging.
a method of imaging whereby only half of k space in the phase encoding axis is filled during acquisition, and the other half is calculated by the MR system
Define: u. 2D k-space filling.
a method of imaging whereby the first line of k-space for each slice is filled, then the second line for each slice is filled, and so on until all lines are filled
Define: v. 3D k-space filling.
a method of imaging whereby an entire volume of tissue is excited by the RF transmission, and slices are divided up after image acquisition
Define: w. centric ordered k-space filling.
filling the central phase encoding steps of k space first, then filling toward the outer edges
Define: x. Fourier transformation.
the mathematical calculation that converts waveforms into discrete values that describe signal strength at each frequency
Define: y. field of view (FOV).
the area of the volume to be imaged
Define: z. slice dephasing.
the dephasing of spins within a slice that is caused by the gradient magnetic field
Define: aa. slice profile.
the shape of the transmitted RF pulse
Define: aa. slice profile.
the shape of the transmitted RF pulse
explain the relationship between gradient magnetic fields and the local magnetic field experienced by tissues along the gradient.
where the gradient add to the primary magnetic field, the tissues experience a higher local magnetic field, where the gradient subtracts from the primary field, the tissues experience a lower local field strength
explain the relationship between gradient magnetic fields and the precessional frequencies of spins along the gradient.
where the gradient add to the primary magnetic field, the tissues experience a higher frequency, where the gradient subtracts from the primary field, the tissues experience a lower frequency
explain “steep” and “shallow” gradient slopes and how they are used to describe the change in magnetic field strength over the length of the gradients.
a steep gradient is one in which the magnetic field strength changes a lot over the length of the gradient, a shallow gradient is one that changes little
- identify which physical gradient coil pair(s) perform(s) slice selection during axial, sagittal, coronal, and oblique imaging in a superconducting MRI system.
axial- slice select is the Z gradient
sagittal- slice select is the X gradient
coronal- slice select is the Y gradient
oblique- slice selection is performed by more than one physical gradient
- describe how the slice select gradient slope affects slice thickness.
a steep slice select gradient produces thinner slices, a shallow slice gradient produces thicker slices
- describe how the RF transmit bandwidth affects slice thickness.
a narrower (decreased) transmit bandwidth produces thinner slices, a wider (increased) bandwidth produces thicker slices
- identify the logical gradient that is on during RF transmission.
slice select
- identify the logical gradient that is on during MR signal reception.
frequency encoding or read-out
- identify the logical gradient that is on before the refocusing pulse.
phase encoding
- state the Nyquist theorem.
a waveform must be sampled at least twice per cycle to represent it accurately
- explain the relationship between RF receive bandwidth and sampling rate that is necessary to satisfy the Nyquist theorem.
they must change proportionally- e.g. as bandwidth increases, sampling rate must increase in order to sample accurately
- explain the relationship between sampling time and sampling rate that is necessary to satisfy the Nyquist theorem.
they are inversely proportional- e.g. as sampling time decreases, sampling rate must increase; as sampling rate decreases, sampling time must increase, etc.
- explain the relationship between sampling time and RF receive bandwidth that is necessary to satisfy the Nyquist theorem.
they are inversely proportional- e.g. as bandwidth increases (wider), sampling time may decrease, as bandwidth decreases (narrower), sampling time must increase. etc.
- describe a “fine” and “coarse” image matrix.
a fine matrix divides the image matrix into smaller pixels; a coarse matrix divides the image matrix into larger pixels
- explain how the numbers of phase and frequency encodings affect the image matrix.
a fine matrix contains many frequency and/or phase encoding steps, a coarse matrix contains fewer frequency and/or phase encoding steps
- identify the number of lines of k-space that are phase encoded during each TR period.
one
- describe how a change in the number of phase encoding steps affect the imaging time of a sequence.
as the number of phase encoding steps increases, the scan time _____ proportionally
- describe how a change in the number of phase encoding steps affect the imaging time of a sequence.
as the number of phase encoding steps increases, the scan time increases proportionally
- identify the area of k-space in which the highest amplitude of MR signal is placed.
the center of k-space, or the shallow phase encoding steps
- identify the area of k-space that determines image resolution.
the outer edges, or the steep phase encoding steps
- identify the area of k-space that determines tissue contrast.
the center, or the shallow phase encoding steps
- explain how RF transmit bandwidth and sampling time affect slice profile.
the narrower (lower) the bandwidth the squarer the slice profile; the longer the sampling time the squarer the slice profile
- explain how changes in RF transmit bandwidth affect the duration of the RF pulse.
a narrower (lower) bandwidth requires a longer RF pulse duration, a wider (higher) bandwidth allows a shorter RF pulse duration.
- explain how changes in RF receive bandwidth affect the minimum TE and TR of a pulse sequence.
an wider (higher) receive bandwidth makes a shorter TE and TR possible, a narrower (lower) receive bandwidth makes a longer TE and TR necessary
- explain how changes in RF transmit bandwidth affect the minimum TE and TR of a pulse sequence.
a wider (higher) transmit bandwidth makes a shorter TE and TR possible, a narrower (lower) transmit bandwidth makes a longer TE and TR necessary
- explain how changes in sampling rate affect the minimum TE and TR of a pulse sequence.
a faster sampling rate makes a shorter TE and TR possible, a slower sampling rate makes a longer TE and TR necessary
- describe the effect that the frequency encoding gradient and RF receive bandwidth have on the field of view (POV).
the FOV can be decreased by using a steeper frequency encoding gradient or a narrower (lower) receive bandwidth (or both); the FOV can be increased by using a shallower frequency encoding gradient or a wider (higher) receive bandwidth (or both)
- identify which ai: as of k-space are filled with steep and which are filled with shallow phase encoding gradients.
the outer edges are filled with steeper phase encoding gradients; the center of k-space is filled with shallow phase encoding gradients
- calculate pixel size of an image when given the POV, number of Frequency Encoding points, number of Phase Encoding steps, and slice thickness.
formula [POV (FE)/# FE points] x [POV (PE)/# PE steps] x [5mm slice thickness]
example [256mm / 256] x [256mm / 256] x [5mm] = 1mm x 1mm x 5mm
- recognize the following on a pulse sequence diagram:
a. frequency encoding gradient.
b. rephasing lobe of the frequency encoding gradient.
c. phase encoding gradient.
d. slice select gradient.
e. rephasing lobe of the slice select gradient.
- recognize the following on a pulse sequence diagram:
a. frequency encoding gradient.
b. rephasing lobe of the frequency encoding gradient.
c. phase encoding gradient.
d. slice select gradient.
e. rephasing lobe of the slice select gradient.
