MRI Data Acquisition

Gradient coils

create a range of precessional frequencies across the excited field of view

linear varian

Magnetic field gradient coils produces ______________ in overall magnetic field strength

conductors

Magnetic field gradient coils are _________ that cause linear change in magnetic field strength along their axes when a current is passed through them. The amount of current passing through the coil determines the amplitude, strength, and slope of gradient.

0.3 gauss/cm

Typical gradient is ________ along the axis perpendicular to the slice plane for a typical field strength of 1.5T

• Maximum/peak gradient strength

• Rise time

• Gradient slew rate

• Duty cycle

Specifications for gradient

Maximum/peak gradient strength

Quoted in units of millitesla per meter (mT/m)

30-45 mT/m

Peak gradient strength for 1.5T to 3.5T field strength

15-25 mT/m

Peak gradient strength for 0.5T field strength

Rise time

• Measured in milliseconds

• Time required to achieve maximum amplitude

Gradient slew rate

Rise time scaled according to the maxim achievable gradient strength

peak gradient strength / rise time

Slew rate =

faster

Higher slew rate contributes to ________ imaging

Duty cycle

Percentage of time gradient at maximum amplitude

gradient field

To examine a specific slice only, a second magnetic field is superimposed on the external magnetic field which has different strength in varying locations. The additional field is called ___________ and is produced by gradient coils.

slice selection gradient

The gradient field that enables us to examine a specific slice is also called ________________

1. Controlling the bandwidth of the RF pulse

2. Controlling the gradient slope perpendicular to the slice plane

Two ways to identify slice thickness

thicker

The wider the range of frequencies, the _________ the slice in which protons will be excited

thinner

For a fixed bandwidth, the stronger the gradient, the greater variation of precession frequency in space and the _______ the slice

thin

______ slices require either a steep slice select gradient slope or a narrow transmit bandwidth

thick

_______ slices require either a shallow slice select gradient slope or a broad transmit bandwidth

90

Once the slice is selected, the signals of the individual voxels must be distinguished with each other.

This can be achieved by applying two gradient fields ___° to each other.

phase difference

A _____________ is created by temporarily changing the spinning rate of the magnetization of one voxel with respect to one another.

frequency encoding gradient

The _______________ is on at the time of the echo event, when the signals are actually being produced.

readout gradient

The frequency encoding gradient is on at the time of the echo event, when the signals are actually being produced.

Since the signals are readout during this time, this is also known as ____________

phase encoding ; frequency encoding

• Frequency and phase encoding are in fact very closely related

• The main difference between the two methods is that the _____________ is completed before we start to measure the signal, whereas the ___________ is applied during the measurement process.

Fourier Transform

MR signals are collected by the machine and decoded later using a computer program called ______________

Slice gap or skip

The space between slices

multi-slice mode

By using the __________, an entire set of images can be acquired simultaneously.

K-space

Area where data collected from the signal are stored

raw data

K-space is the “________” for magnetic resonance imaging.

It is a spatial frequency in domain, where information about the frequency of  a signal and where it comes from the patient is collected and stored

3D Volume Acquisition

Has the advantage of being able able to produce thinner and more contiguous slices

phase encoding

In volume imaging, __________ is used to create slices in addition to creating the voxel rows

quadrature demulation ; phase quadrature technique

Quadrature detection is also called ________________ or ___________

• Real channel

• Imaginary channel

2 components of quadrature detection

Real channel

“I”, “in-phase”

Imaginary channel

“Q”, “quadrature”

quadrature detection

_____________ offers an increase in SNR by a factor of 2 = 1.41 over detection by a single linear receiver coils. 

• as a “real” image

• as an “imaginary” image

• as a magnitude image

• as a phase image.

The signal data from quadrature detection may be reconstructed in several ways:

Fourier transform

Gives the corresponding signal intensities a certain value specific to the signal and its location, which results in MR image.

• Main field (B0) coil

• Shim coils

• Gradient coils

• RF coils

• Patient coils

Imaging coils form outer to inner

Main field (B0) coil

Principal magnet windings

Shim coils

To improve homogeneity

Gradient coils

For imaging, including active shields

RF coils

Transmits the B1 Field

Patient coils

Primarily to detect the MR signal, some are transmit/receive

• Volume Coil

• Surface coil

• Phased array Coil

Types of coils

Volume coil

• Typically surrounds either a whole body or a specific region

• Provides homogenous B1 field 

less

In volume coils, SNR is _____ compared with surface or phased array coil

Surface coil

• Placed on the surface of a region of interest to acquire images with very high SNR 

• High SNR is obtained at the cost of limited volume coverage and decreasing B1 homogeneity with distance from the coil 

• Receive only mode

Phased array Coil

• Consists of two or more geometrically aligned surface coils used in conjunction 

• Combines the advantages of surface coil  (high SNR) and volume coil (large FOV) 

less

The closer a coils is to the area to be excited, the _______ RF energy needed to create transverse magnetization

more

The closer the receiver coil to the excited volume, the _____ signal is detected