Scan Principles Image Formation

Upon being placed in an MR scanner, a large majority of the patient’s hydrogen protons

align themselves with the direction of the static magnetic field (B0) and its natural alignment.

the net magnetization vector

The total number of excess protons aligned with the magnetic field

The total number of excess protons aligned with the magnetic field is proportional to

magnetic field strength

The signal produced immediately following an RF pulse is

a FID (Free Induction Decay) signal

fourier transform

The process that digitizes the MR signals

Raw data is a file on the computer where the collected echoes are stored prior to being processed into an image by

Fourier Transform.

k-space

the storage location of MR signal data

Referring to k-space, the data containing high resolution are located _____

along the outer lines

Referring to k-space, the data containing signal and contrast are located _____

along the central lines.

Each line of K-space is defined by

the phase encoding gradient

elliptic centric k-space filling

Collecting the low frequency (high amplitude signal) data points in k-space at the start of the scan (in a spiral fashion)

During dynamic enhanced imaging for vasculature or visceral structures, contrast is administered and k-space is filled with _____. why

centric k-space filling to ensure that the contrast enhancement is well visualized

2D acquisitions usually require

an interslice gap

Long TR's are ___ advantageous in a 3D acquisition

not

3D sequence provides____ SNR and _____ of crosstalk artifacts

greater
elimination

The overall (SNR) will increase in 3D volume imaging when there is increase in either

the Field of View (FOV) and/or an increase in the number of slices or partitions (chunks).

The option to reconstruct multiple planes from a single data set is advantageous in a 3D pulse sequence due to

the thin slices used in 3D sequences and the absence of an interslice gap.

Reconstruction algorithms of 3D data sets include

MPR, MIP

MPR

Multi-Planar Reconstruction, the ability to generate images in scan planes in addition to the acquisition plane, yielding multiple slice plane orientations from one acquired data set

MIP functions

Maximum Intensity Projection (MaxIP)

Minimum Intensity Projection (MinIP)

Maximum Intensity Projection (MaxIP)

A reconstruction algorithm utilizing bright pixel intensities to create collapsed images of a data set; i.e., bright blood MRA’s such as TOF, PCA, CE-MRA, MRCP, etc.

Minimum Intensity Projection (MinIP)

A software algorithm utilizing dark or hypointense pixel intensities, creating reconstructed images such as black blood angiography

The TSE factor (echo train length) is a ________ and can lead to ______

scan time acceleration factor,
 increased thermal induction in patients due to the increased RF power

In a Fast Spin Echo sequence, the effective TE are ____________.
This would occur at the line that is ________,
and may occur __________

the echoes that are encoded with a low amplitude phase encoding gradient.
closest to the center of k-space (k0)
at the beginning or end of the shot, depending on the profile order.

gradient

defined as a small change in the magnetic field along a particular axis

narrow receiver bandwidth _____ susceptibility artifact, and should _____ be used in the presence of metal implants.

increases, never

Lower receiver bandwidth = ____ signal-to-noise = ____ chemical shift

higher, higher

wide receiver bandwidth does what

generates lower signal-to-noise ratio images
should be applied in anatomical regions that contain high fat and water interfaces
is utilized with a high readout gradient amplitude

Transmit Bandwidth done when and affects what and is in what range

Occurs during transmission describing properties of RF pulses, typically in the range of a few kilohertz (kHz),
RF excitation pulse required for slice selection

Receive Bandwidth done when and affects what and is in what range

Applied during reception of the MR signal, has profound effect on overall SNR, susceptibility, typically in the range of 16-64 kHz

Transmit Bandwidth aka

pulse bandwidth

RF excitation pulse required for slice selection

Transmit or pulse bandwidth

Slice thickness is _____ to the bandwidth of the RF pulse

proportional

Lowering the pulse bandwidth can ____ the slice thickness

reduce

During transmission, RF pulses contain ________ to __________

a range of frequencies (bandwidth),
excite a specific slice thickness or volume

The time the RF pulse is on

reflects the type of sequence selected

times the RF pulse is on

Low SAR: ±3.5ms
Normal mode: ±2.5ms
Fast mode: ±1.2ms

Low SAR times on

±3.5ms

Normal mode times on

±2.5ms

Fast mode times on

±1.2ms

Low SAR mode characteristics

Longer RF pulse with good slice profile 
Reduced SAR values (lower amplitude)
Less crosstalk between slice, narrower gaps allowed
Longer minimum TE’s and TR’s

Normal mode characteristics

Normal RF pulse with good slice profile
Optimized SAR behavior

Fast RF mode characteristics

Shorter RF pulse, with a compromised slice profile

Higher SAR compared to the other modes (higher amplitude)

Shorter echo spacing (ES)

Opportunity for shorter TE’s and TR’s

Fewer susceptibility artifacts

what mode typically results in more noise and increased potential for peripheral stimulation

fast mode

fast mode typically results in

more noise and increased potential for peripheral stimulation

receiver bandwidth (rBw)

the range of frequencies accepted by the receiver to sample the MR signal, Describes which frequency range from our analyzed echo signal is

transferred into pixels

which badnwidth can be adjusted at console

reciever BW

what describes which frequency range from our analyzed echo signal is transferred into pixels

rBW

acquisition bandwidth

rBW

acquisition bandwidth has ____ relationship to SNR

direct

decreasing Receiver bandwidth by factor of 2 _____ SNR by

increases by √2 (approximately 40%)

advantages of increasing rBW

\/ the sampling time Ts

\/ sequence timing, allowing shorter TR and TE values

\/ echo spacing (less blurring)

\/ susceptibility artifacts

\/ chemical shift artifact

\/ metal artifacts

/\ available ETL

disadvantages of increasing rBW

/\ noise sampled due to larger frequency range

\/ SNR

/\ minimum-required Field of View due to larger frequency range

advantages of decreasing rBW

\/ noise sampled due to smaller frequency range

/\ SNR 

\/ minimum-required Field of View due to smaller frequency

range

disadvantages of decreasing rBW

/\ the sampling time Ts and

/\ the sequence timing, creating longer minimum TR and TE

values

/\echo spacing (more blurring)

/\ susceptibility artifacts

/\ chemical shift artifact

/\metal artifacts

Transmit Bandwidth is Enabled during ____ and affects

RF, slice thickness

Receive Bandwidth is range of

frequencies collected during frequency encoding

Wide rBw causes

/\ data acquistion, \/ chemical shift artifacts, but /\ noise

Halving rBw ____ SNR by

improves ~40%

Narrow rBw causes

/\ chemical shift and motion artifacts, # of slices per TR \/

In order to generate an MR image, the slice select gradient must be turned on during

the RF energy application

The slice selection gradient is the determinant of

scan plane, slice thickness

The slice selection process determines:

Slice orientation
Slice thickness
Slice position

A steep slice select gradient slope yields ___ slice thickness

thin

A ____ slice select gradient slope yields thin slice thickness

steep

decreasing the slice selection gradient strength will ______ the slice thickness of the resulting slice.

increase

A thin slice thickness is achieved through the utilization of

a steep, high amplitude slice select gradient, and a narrow transmit bandwidth (tBW)

The transmit bandwidth of the RF pulse affects the

slice thickness, resolution and the overall SNR in the MR image

A steep gradient is applied with the transmit bandwidth at _____ to ______

the Larmor frequency of H (hydrogen) in order to achieve a thin slice thickness in MR imaging.

what must remain constant to maintain slice thickness

The range of transmitted frequencies at each slice position

The gradient that is on during the production of the echo

the frequency encoding (readout) gradient.

The readout gradient is usually turned on during _______ and _________

the sampling or readout of the peak echo, during frequency encoding

sampling time

the time to sample the entire K-space line

controller of sample time

frequency / readout gradient

sampling time can be calculated by

frequency matrix (number of samples) ÷ rBW

sampling interval

time between sampling points

sampling rate

signal sampling frequency

According to Nyquist theory, our sampling rate must be

twice that of our sampling frequency in order to avoid aliasing in our image

As the receiver bandwidth increases, the sampling rate or frequency ______

increases

the sampling rate is _______ to the receiver bandwidth

proportional

Increases in the receiver bandwidth results in ______ in sampling rate

increases

why we don't get wraparound artifacts (aliasing) in the frequency direction

we sample twice per cycle bc Nyquist theorum

why we get wrap in phase direction

no sampling twice per cycle with the phase gradient

sample twice per cycle means

twice the highest frequency in the receive bandwidth

if we utilize a sampling time of 8ms with a frequency matrix of 256, we need to sample at a frequency of 32000 Hz (once every 0.00003125 s) to acquire 256 data points during the sample time.

the rBW is reduced (narrow receiver bandwidth), the resultant effect would be an

increased sampling time, thus an increased minimum TE at the operator console.

The resultant effect on the image would be more chemical shift artifacts (increased sampling time) but an increased SNR due to less noise sampled.

gradient moment nulling aka

flow compensation

using gradient moment nulling requires

is required that a longer minimum TE is used due to the use of additional gradients

Gradient moment nulling (flow compensation) is used to

compensate for first order motion and slow flowing vessels.

first order motion

protons moving with constant velocity

Single-order gradient moment nulling only compensates for

first-order motion, such as flow with constant velocity

Increasing the field of view (FOV) would yield a ___ in spatial resolution (detail) in the image due to an _____ in pixel size

decrease, increase

what affect the flip angle

The strength and duration of the RF field

as flip angle is increased

SNR (signal to noise ratio) increases to a point called the Ernst angle.

Ernst angle

optimal flip angle that yields the maximum signal for a particular spin in the least amount of time.

example of TR and slice numbers

If the set TR allows for 10 slices, 3 acquisitions will be required to obtain the entire

26 slice group. Raising the TR to allow for 14 slices would permit the slices to be

acquired in two acquisitions, for example.

true mechanisms of saturation

Full saturation suppresses longitudinal magnetization; occurs when NMV is flipped a full 180°
Partial saturation yields T1 weighting; occurs when NMV >90°
A pre-saturation pulse excites hydrogen protons to a frequency that gives no signal on the image (this occurs prior to the excitation pulse)

Full saturation suppresses _____; occurs when

longitudinal magnetization, NMV is flipped a full 180°

Partial saturation yields _____, occurs when

T1 weighting; NMV >90°