4. Pulse sequences are a necessary
part of MRI:
• Dephasing because the magnetic
field has inhomogeneities
• Regenerate signal
• They determine if an image is T1,
T2, PD weighted.
Definition: a series of RF pulses, gradient applications and
intervening time periods.
You basically are wanting to achieve a specific
contrast quickly while not having artifact and losing
signal to noise.
5. With a pulse sequence you want
to acquire one of these contrast
weighted images.
PD
6.
7.
8. These are your main
types of pulse
sequences and can vary
depending if the
technologist wants to
speed it up or change
the image contrast.
10. RF pulse application
Slice select or Z
gradient
Phase Encoding or Y
gradient
Frequency encoding,
readout or X gradient
11. • The most common pulse sequences used
• Uses a 180° pulse to regenerate signal as
tissues are relaxing and desired tissue
contrast needs to be attained.
• The time between the 90° pulses is TR
• TAU: the time after the 180° RF pulse=
time to dephase when the 90° RF pulse is
withdrawn.
• Can use one or two echo’s to acquire T1,
T2 or PD weighted images
• Take the longest scanning time because
only one line of k-space is filled.
• Sharper imaging capabilities than Fast
Spin Echo.
12.
13. Protons spinning on their
axis along the B0
90º pulse directs it along
the transverse plane (x,y)
180° pulse is applied to
rephase the signal.
Signal decays because of
inhomogeneities in the
magnetic field. T2* dephasing
happens.
The slow and fast spins
catch up.
Transverse magnetization
in phase occurs and the
coil has signal or spin
echo.
15. • The most common pulse
sequences used.
• Uses multiple 180° refocusing
pulse per TR.
• Can use two or more echo’s to
acquire T1, T2 or PD weighted
images.
• (FSE)Fast Spin Echo times have
shorter imaging times than SE.
• High resolutions as one line of
k-space is filled per echo.
• Sometimes referred to as Turbo
Spin Echo (TSE)
16. • A completion of more than 1 phase
encoding step as well as filling more
than one line of k space per TR to
reduce scan time. Here is where
your echo train length comes into
play.
• Each 180° refocusing pulse per TR =
# of echoes and lines of k space
filled.
• Weighting is achieved by effective TE
and shallow & steep gradients
working together with k space.
ECHO
TRAIN
How many echo train
lengths do you see
here?
17.
18. • All lines of k space are filled in a single TR, by making use of Half Fourier
Imaging.
• SAR is a major factor
• Reduced scan times and number of allowed slices inhibiting technologist ability
to cover anatomy in a single shot.
• Useful in cardiac and breath hold imaging
19. • Applies a reverse flip angle (-90
degree) excitation pulse at the end
of the echo train to cause the
transverse magnetization into the
longitudinal state to be ready for
the next TR period.
• Shortens T1 relaxation wait times.
= short TR or scan time reduced
• Best when imaging T2 and proton
density
• Patients usually become heated
quickly
90° 180°180° 180°180°
-90°
20. • Use when you need heavy T1
weighting
• Can be used with a SE or GRE
sequence.
• Begins with a 180° RF pulse
that inverts the magnetization
followed by a 90° RF pulse
that brings the residual
longitudinal magnetization
into the x-y or transverse
plane where it can be
detected by an RF coil.
• TI: the time between the initial
21. (STIR) short tau
inversion recovery
• can achieve fast
suppression
techniques.
• Short TI-suppress
fat
• Don’t use when
gadolinium is
given.
• Lesions in bone
like tumors and
bruising are seen.
(FLAIR) fluid
attenuating
inversion
recovery
• can achieve fast
suppression
techniques.
• Nulling water
• Long TI
• Mainly best to
visualize cord
lesions, brain
plaques, etc.
22. • Use gradients fields to achieve
transverse magnetization instead of
180°.
• Makes use of flip angles less than
90° to achieve transverse
magnetization.
• Commonly used to achieve T1, PD &
T2*
• T2 relaxation is referred to often as
T2*
• Signal is sampled at TE
• Used to view hemorrhages
23.
24.
25. 1. Steady State or
Coherent
2. Spoiled or
Incoherent
3. Balanced 4. EPI/Fast
Gradient
26. • Known as steady state as you are achieving stable
conditions after the hydrogen molecule is excited
it suffers a loss of energy with energy transfer
occurring.
• TR controls how much energy is lost so keeping it
less than 50ms is ideal.
• The energy going in is controlled by the flip angle
and to keep it stable an ideal reduced flip angle is
30° -45º.
• A rewinder is applied to maintain your transverse
magnetization and allow for the signal from longer
T2* spins.
• Transverse magnetization will exist from TR to TR
• Shorter TR than TI and T2*
• CSF is going to be bright no matter what flip angle
is chosen
• Great for cardiac, fetal and abdominal MRI
27. • Known as incoherent
• You do not want any residual
transverse magnetization between
pulses so you will use spoilers right
before an RF pulse is applied.
• A large flip angle = more T1
weighting
• Example in and out of phase imaging
for abdominal imaging.
• Used in 2D to acquire T1 breath Hold
images & vessel imaging
• Spoiling- dephasing magnetic
A B
RF Spoiling
RF is applied to the slice
at a specific frequency
and phase then the
receiver coil speaks with
the transmit coil resulting
in only the frequencies
from the most recent
echoes to be digitized
and only then affect
image contrast.
Gradient Spoiling
• Total opposite of th
function of the
rewinder.
• Dephases residual
magnetization so th
phase is not stable
or ready at the next
repetition.
• Less T2* and very
strong T1 effects.
28. • Known for short TR and a gradient
refocusing
• You don’t have to worry about T2*
effects compared with other GRE & you
acquire a heavier T2.
• TE is longer than the TR
• 2 TEs: Actual TE and Effective TE
• Better contrast to noise (C/N) and
signal to noise (S/N) ratios than other
GRE
• Better image contrast because you
achieve high SNR
• Great for vascular imaging such as
cardiac MR
• Effective TE=(2 x T)-TE (time to
RF
Gz
Gy
Echo
Gz
TR
TE
TR
T2
FID 1 SE 1
Opposite
gradients
Phase
1
Phase 2
Rewinder 1
Rewinder 2
𝑎2
a⁴a¹ a³
29. Modification of the coherent gradient echo sequence discussed
before, but makes use of a balancing gradient to correct phase
errors because of flowing blood and CSF fluid and an
alternating RF excitation scheme to enhance steady state
effects.
• The difference b/t a tissues T1 and T2 time determines how
bright the signal will be. The greater the time difference the
brighter the signal.
• Higher flip angles with shorter TR
• TE is always half the TR = short TE time
• Initial used for heart and great vessel imaging techniques,
30. • Great for rapid imaging techniques
• It applies only a portion of the RF pulse and a portion of
the echo is read.
• TE is kept minimal allowing TR and scan to be reduced.
• Great when you want to get multiple images in a single
breath hold without respiratory motion or post contrast
dynamic imaging.
• You want to fill a line of k-space in one TR period by
using a train of echo’s not a train of 180° RF pulses.
31. • Known for quick imaging
• Gradients must be strong
and turn off and on
quickly
• Single or multi shot
sequences that fill k-
space with data from
gradient echoes in one
shot.
• Mainly used in diffusion,
perfusion and functional
MRI
32. Objective
Understand the basics of k-space
filling and mapping
Understand the role of (FFT) Fast
Fourier Transformation
Define post processing techniques
33. K-Space: is where
collected data is stored in
the array processor as
data points, which are
stored in k-space.
• Rectangular
• Has two axes (phase &
frequency)
• Data point contain info
for the whole slice.
• Measurements unit is
called radians per cm. FREQUENCY AXIS
PHASEAXIS
35. • Once it is filled with data from
signal and resolution an image is
displayed.
• Image quality is determined on
how k-space is filled.
• Central portion: high signal
amplitude and low resolution;
mainly for contrast of image
• Outer portion: low signal
amplitude and high resolution;
mainly for spatial resolution
• Looks like a
grid
• Each data
point in k-
space maps…
To every
point on the
MR image.
36. • What sequence you choose
determines how k-space is filled
• For example Cartesian would
generally be used for gradient and
spin echo pulses.
• Linear (most common): horizontal
filling starting at the bottom and
going to the top.
• Centric: middle line first is
acquired working it’s way to the
top and bottom.
37. • Filled line by line beginning to fill the center lines first.
• This method applies the shallowest phase encoding steps and at the
end of the sequence the steep phase encoding steps are applied.
• Best used with fast gradient echo technique imaging.
1
2
4
Filled last
3
Filled first
Filled last
38. • Begins in the middle and fills out only during a specific part of
the sequence not the entire sequence, as low spatial
frequencies are in the middle.
• Best when wanting acquire CE MRA imaging because contrast
media has high signal intensity data
1
2
4
5
3
39. • Used for dynamic MR imaging
• The same image is collecting into k-space many times as
contrast goes in and out . The purpose is to see how tissue
respond to contrast over a time period.
• The center of k-space will contain the important information.
• Best when you want to acquire fast high resolution imaging
40. • All lines of k space are filled in one repetition linearly. (
“single shot” (SS)
• All data is collected either by a EPI train of gradient
echo's or a echo train by means of a fast spin echo
sequence.
• Readout gradients switch quickly from positive to
negative filling k space either left to right or right to left
and is done until it reaches the central portion of k space
the only negative polarity is used to filled the lower
41. • The most complex of filling techniques
• No TR used and k space is filled in one go round
• Goal is for rapid filling of k space and enhance
the filling of the central portion of k space.
Varied types of spiral k space filling are:
1. Elliptical
2. Propeller
Two multi shot methods increase time but better
image quality:
1. K space segmentation by acquisition
2. K space segmentation by echo's
Two types of gradient power supply modifications
that can be used are:
1. Resonant power supply
2. Non Resonant power supply
42. Factors determining contrast of
image:
• Gradient Echo EPI: any flip
angle followed by EPI readout
gradient echo’s = 1 TR pass to
acquire the images
• Spin echo EPI: 90 degree
excitation pulse then 180
degree refocusing pulse then
EPI readout gradient echoes =
longer scan times, but better
image quality and increased
heating to patient.
Hybrid sequences:
• Combo of gradient and spin echo
sequences (GRASE).
• Make use of the speed of the
gradient and the better image
quality of the spin echo sequence.
USES:
• Functional research MRI
• Cardiac
Limitations:
• Nerve stimulation, severe acoustic
noise as well as artefacts and
distortion.
45. •All protons in the tissue align parallel to the direction of the magnetic
field.
•The protons are excited to a higher energy state, using a radio
frequency electromagnetic pulse.
•The excited protons return back to the energy equilibrium.
•The accepted energy is retransmitted back and picked up by the coil
system.
•This energy is passed to the channel receiver and can be measured.
•A K-space image is formed, measuring the retransmitted signal
•The real image of the patient is obtained by Fourier transform of the K-
space image
•The signal is received by a simple coil with quite homogeneous
sensitivity over the whole imaged volume.
•The signals are received simultaneously by several receiver coils with
varying spatial sensitivity. More information about the spatial position
of the MRI signal.
•Parallel imaging speeds up the acquisition of the data.
This advantage is useful for:
1.The ability to image dynamic process without major movement
artifacts.
50. •Perfusionis physiologically defined as
the steady-state delivery of blood to an element of
tissue.
• The primary focus is to make contact capillary blood
flow. Used for different physiologic parameters that
also affect the MR signal, e.g., blood volume, blood
velocity, and blood oxygenation.
• Used to image tumors as they have a high
concentration of blood vessels.
51. T2* decay (susceptibility)
• Is basically a decrease in signal strength and seen in the Free
Induction Decay (FID) after the RF pulse.
• Inhomogeneities exist in the magnetic and cause dephasing. They
exist because of the patient magnetic makeup and the imperfect
construction of the magnet = T2* decay
• Since it exist and nothing can be done about it technologist can use
it in imaging. Gradient echo imaging
such as SWI sequences work off T2*
decay.
• Visualizing calcium and blood in the
brain is ideal.
56. Objective
Understand the reason to use
MRA
MRV
Know what dynamic imaging is and when to use it
Understand Flow phenomenon and its effect and
uses in MRI
Understand contrast bolus detection and why it is
used
58. Types of flow phenomena: flowing
nuclei that cause phase ghosting
artifacts.
1. TOF or Time of Flight: either flow
related enhancing or high velocity
signal loss.
Flow related enhancement increases
when:
TE & Velocity flow decreases
Slice thickness increases
High Velocity signal void increases when:
TE & Velocity of flow increasing
Slice thickness decreasing
2. Entry Slice Phenomena increases:
at first slice in the stack
Long TR is used
With thin slices & fast flow
When there is counter current flow
3. Intra- Voxel Dephasing:
Flow affects the image quality
TOF effects give signal void or
enhancement
Entry slice phenomenon effects give a
different signal intensity to flowing
nuclei
Lumen signal intensity is affected by
flow
59. • Called “TOF”
• Enhancement is based on
flow of blood.
• As the blood enters the
image only a small amount of
excitation pulses hit it so it is
not saturated and gives off
higher signal than the tissues
around it that are saturated.
• Can be acquired with 2D
(slice by slice) or 3D (volume)
acquisitions.
Four main types of flow:
1. Laminar: different but consistent
velocities across the vessel. Center
flow is fastest.
2. Spiral: the direction of blood flow is
spiral
3. Vortex: starts out laminar then
changes after going through a
stenosis in the vessel. Center has
a high velocity, spirals at the walls.
4. Turbulent: has different velocities
and fluctuates at random.
60. Methods of
Reduction:
1. Even Echo
Rephasing:
Works off the principle
that if the second echo
is given the same
amount of time to
rephrase as the first
echo was given.
Reduces artifact in T2
weighted images.
2. Gradient
Moment
Nulling:
Compensates the
phase values of
flowing nuclei along
a gradient.
Works best on slow,
laminar flow within a
slice.
3. Spatial Pre-
Saturation:
61. • Related to the change in the
velocity of blood in phase shifts.
• Provides information about
vascular anatomy, flow speed,
multidirectional blood flow and
flow direction.
• To avoid in-plane flow
artifact, CE is the standard for
evaluation of neck, chest,
body and peripheral vascular
systems.
• T1 3D gradient Echo is used
62. • “Maximum Intensity Pixel”
• Source images are stacked and
the computer uses an
algorithm that projects through
the stack.
• The result is the darkest or
brightest pixel along that ray’s
path.
• This is not the image of the
vessel simply the signal of the
vessel.
• An unlimited number of views
63. • 3D images that are
stacked and
projected however
you choose.
• Technologist can
choose if they want
a sagittal, coronal or
axial plane to avoid
overlapping.
• An unlimited number
of views just from
one data set.
64. • Mainly used in breast and
angiography exams
• Unenhanced T1-weighted
sequence is digitally
subtracted from the
identical sequence
performed after gadolinium
administration.
• Excellent in evaluating
hemorrhagic masses or
lesions with high signal
intensity on unenhanced
T1-weighted sequences,
such as complicated cysts
65. Requires rapid MR acquisition after
the administration of contrast agent.
66. Fluoro-Triggering
• Technologist is able to
watch as contrast enters
and is able to begin at
their desired time.
• Known as CAREBOLUS,
BOLUS TRAK,
SMARTPREP
67. Automatic Bolus
Detection
• MRI scan begins when the
contrast reaches the area of
interest.
• A red tracker is placed in the
area (educational purposes)
and once signal is increased the
scan begins automatically.
68. Timing Test Bolus
• No accurate estimation of time is necessary.
• A small amount of gad is injected with scans being
repeated to calculate the exact time to start scan.
70. Objective
Understand how parameters a technologist uses can and will
affect the image.
Understand what you will trade off for when you make a
change to a parameter and it’s affect on the image.
Those parameters affected are:
1. TR
2. Flip angle
3. TE
4. Signal to noise (SNR)
5. Contrast to noise (CNR)
6. Spatial resolution
7. Scan time
Image Quality is
controlled here
Image Weighting
is controlled here
71. Contras
t to
noise
(CNR)
Signal
to noise
(SNR)
Spatial
resolutio
n
Acquisition
time
Defined as the
difference in
SNR between
two adjacent
areas. The
technologist
is able to see
high & low
signal areas.
The ratio of the
amplitude of
the signal
received to the
average
amplitude of
the noise.
Basically, how
grainy and
Ability to
distinguish
between two
points as being
separate and
distinct. Voxel
size is affected
by this
parameter.
How long it
takes to acquire
the total data
or fill k space.
Long scan
times give
patients more
opportunity to
move.
72. Factor
Affecting SNR
• Magnetic Field
Strength
• Proton density of
examination area
• Voxel Volume
• TR, TE, & Flip
angle
• NEX
• Receive
bandwidth
• Coil type
How much noise you have in relation to the
signal in an image.
73. • Anytime you increase the
field strength of a magnet
you increase your SNR.
• Note that at a lower
strength magnet you can
increase your SNR by
changing other factors, but
this will significantly
increase your scan time.
LOW FIELD
SCANNER
HIGH FIELD
SCANNER
74. As learned earlier certain
parts of the body have more
protons than others.
• The pelvis has high proton
density areas so it yields
high SNR.
• The lungs have low proton
density so it yields low
SNR.
75. • A digital image is built off a pixel.
• A voxel is a patients tissue made
up by a pixel & slice thickness.
• Pixel area=FOV dimensions/matrix
size
76. Coarse Matrix: low number of frequency encodings or maybe phase
encodings gives you a low number of pixels in your field of view = large
pixels and voxels. Yields a high SNR but poor spatial resolution.
Fine Matrix: high number of frequency encodings or maybe phase
encodings gives you a large number of pixels in your field of view = small
pixels and voxels. Yields a low SNR but better spatial resolution.
• If you change the size of the voxel in anyway you will change the SNR.
i.e. reduce the voxel=reduce SNR
• Change slice thickness: decrease slice thickness decrease SNR
• Change image matrix: increase matrix increase SNR
• Change FOV: increase FOV increase SNR
77. 10 X
10
5 X 5
FREQUENC
Y
PHASE
FINE COARSE
FREQUENC
Y
PHASE
78. Matrix the phase
and frequency steps to
make an image and
are columns of pixels
that form the image.
The number of
pixels/voxel's in our
image, not the size of
our image, which is the
field of view.
Determining factor for
the resolution of an
image.
Increasing the matrix
increases the scan time
& resolution and
79. FOV means the “field of view” and
is the area of anatomy covered in the
image.
Determines the pixel area
Controls the voxel (patient tissue on
the image) volume in two dimensions.
Increasing FOV increases voxel
volume & SNR, decreases resolution
& image size.
82. • Increase TR increases SNR
We experience an increase in longitudinal magnetization
creating transverse magnetization after it is excited.
• Increasing TE decreases SNR
Transverse magnetization isn’t able to rephrase and produce
an echo.
83. Flip Angle (Ernst
Angle)
• Determining factor in signal intensity and
image contrast.
• Used to keep the desired contrast of an
image when you have changed other factors
to reduce scan times that would otherwise
degrade the image contrast.
• Especially helpful when using a gradient
echo, as you won’t use a 180° pulse with
these.
• When you increase the flip angle it becomes
more T1 weighted and vice versa.
• If you increase the flip angle the SNR
increases up until the Ernst Angle
• Determines the amount of T1 and Proton
84. Number of Averages
(NSA)
• Known as NSA, Naq, NEX
(number of excitations),
• This is the number of
times data is collected at
the same amplitude or
speed along the phase
encoding slope
• Increasing NSA will
increase your SNR giving
you better quality images,
increases you time, and
can cancel motion like
breathing artifacts.
• Increases by a factor of 2
85. RECEIVE BANDWIDTH
Bandwidth: the range of frequencies
that are responsible for receiving or
transmission of MR signal.
Information collected as to how we plan to collect
frequency information about the anatomy of
interest.
Used to get more signal to your image and less
noise as well as reduce certain artifacts.
By reducing the bandwidth you will increase the SNR
and the time as well as reduce the noise to the
image. In turn our allowed TE increases, which
inadvertently increases the TR = longer scan times.
By increasing the bandwidth you will allow more
noise in turn decreasing your TE allowed and
inadvertently decreasing the TR we can use =
Receive: this signal is sampled
during the readout gradient being
applied.
86. Quadrature use two
coils and give increased SNR
Surface are always placed
on the surface of the anatomy.
Increase SNR
Phased Array
increase SNR
*Properly placing the coil is highly
important for max SNR. Fig. 4.28
pg.124
Study Summary pg. 123
87. Contrast
to noise
(CNR)
Signal
to noise
(SNR)
Spatial
resolutio
n
Acquisition
time
Defined as the
difference in
SNR between
two adjacent
areas. The
technologist
is able to see
high & low
signal areas.
The ratio of the
amplitude of the
signal received
to the average
amplitude of the
noise.
Basically, how
grainy and
image will be.
Ability to
distinguish
between two
points as being
separate and
distinct. Voxel
size is affected
by with this
parameter.
How long it
takes to acquire
the total data or
fill k space.
Long scan times
give patients
more
opportunity to
move.
88. 1. T2 weighted images: it allows the ability to see a difference in
normal and abnormal tissues like liver tumors (high signal) from normal
tissue ( low signal).
2. Contrast agents: these agent enhance pathology while normal
tissue is not enhanced and has low signal.
3. Chemical pre saturation tech: saturating out normal anatomy,
pathology now can been seen.
4. MTC/Magnetization transfer contrast: used with protons
that have a long T2 time and increases the CNR b/t pathology and
normal tissues in joint as well as angiography imaging.
89. Contrast
to noise
(CNR)
Signal
to noise
(SNR)
Spatial
resolutio
n
Acquisition
time
Defined as the
difference in
SNR between
two adjacent
areas. The
technologist is
able to see high
& low signal
areas.
The ratio of the
amplitude of the
signal received
to the average
amplitude of the
noise.
Basically, how
grainy and
image will be.
Ability to
distinguish
between two
points as being
separate and
distinct. Voxel
size is affected
by with this
parameter.
How long it
takes to acquire
the total data or
fill k space.
Long scan times
give patients
more
opportunity to
move.
90. Spatial resolution determines
how "sharp" the image will
look. Low resolution = fuzzy
edges on the image.
THIN SLICE
THICK SLICE
Determines how much tissue
is sampled and affects the
resolution of the image.
91. 1. Make changes to our slice thickness =
affect resolution. Thinner slices = better
resolution
2. Adjust the FOV for better resolution.
Larger FOV = decreases resolution, but
increases SNR
3. Adjusting the matrix will adjust our
resolution. Decreasing matrix =
decreases scan time & resolution but
increases SNR.
4. Increase our number of averages or NEX
to give more signal to the image, but will
increase the scan time.
5. Or decreasing the receiving bandwidth for
more signal = increase scan time.
6. Now check your If your
92. Spatial Resolution &
Pixel Dimension
Examples:
Rectangular FOV: 256mm X 128mm
Image Matrix 256 X 128
1mm X 1mm pixel²
Square FOV : 256 X
256
Image Matrix 256 X
256
1mm X 1mm pixel²
Rectangula
r pixel
Square
pixel
93. Rectangular FOV
Sometime referred to as asymmetrical
FOV
Best to use on anatomy that will fit in a
rectangle & sustains the spatial
resolution.
It is conveyed as a percentage or
proportion.
The FOV is smaller in the phase
direction with less pixels verses the
frequency direction to reduce the scan
time.
Benefit is the technologist is able to
acquire a matrix with more pixels, but
with the scan times of a matrix with less
pixels.
Example of Rectangular FOV: 256mm X
94. • Effects of Scan time
• Trade offs
• Decision Making
Long scan times allow for chances that a
patient will move during the scan.
2D and 3D imaging: if a patient moves all the
slices will be affected.
Sequential imaging: only the slice being
scanned will have movement.
Goal is imaging with the shortest scan time.
Determine the exam that is needed prior to
placing patient on the table.
Determine the condition and cooperation of
each patient prior to scanning.
Make patients comfortable from the
beginning.
Know your protocol determined by the
radiologist first.
Choose your correct coil and position it
correctly.
FACTORS AFFECTING
SCAN TIME:
1. TR: double TR double
scan time
2. PHASE MATRIX: double
phase matric double
scan time
3. NEX: double the NEX
doubles scan time
95. • Able to see all anatomy in one plane of imaging
as entire volume of tissue is excited.
• Reduction in slice thickness as well as no need
for a slice gap.
• Smaller number of NEX
• SNR quality is best verses conventional imaging.
• Relatively longer scan times are common
• Isotropic or symmetrical voxel = the best
resolution in any plane.
• Uses: orthopedic, vascular, visualize small lesions and some brain
imaging
• Scan time=
• TR x NEX x #of phase encodings x Number of slice encodings
• Example of isotropic voxel: 256 x 256
• Example of anisotropic voxel: 124 x 256
Editor's Notes
So we are pulsing the RF and gradients at specific times and a specific order to achieve T1, T2 and PD contrast MR images.
Several echoes per TR
Cartesian starts from left and goes right.
Spiral: fills from center to forward
Radial: multiple shots starting from the center to the outer edges of k-space. Used with Blade or Propeller imaging
Zig Zag: filled from left to right and back again.
The imaging technique is referred to at times as susceptibility sequence.
A larger box can fit more stuff inside. The larger the voxel the more nuclei you can it into it and vice versa.
