Volume Imaging And Multiplanar Reconstruction

Front 1

2D reconstruction review:
Within one TR period, what process are repeated for all of the slices?

Back 1

1. Each slice recieves an RF pulse at a specific range of frequencies in order to specify the slice position and thickness.
2. Recieves a unique amount of phase encoding during the dollection of each echo.
3. Both frequency and phase encoding are necessary for locating the precise position of signal within an image.

Front 2

What are some drawbacks to 2D imaging?

Back 2

2D imaging is limited by:

1. S/N (Slie thickness is limited by the S/N. To obtain thinner slices, S/N is compromised because fewer protons are contained in thinner slices.
2. There is a restriction due to the maximum slice selection gradient amplitude and by the maximum bandwidth of the RF transmitter amplifier and bandwidth.
3. 2D is prone to cross talk problems. *Contiguous imaging is NOT recommended with a 2D pulse sequence.

Front 3

How do 3D sequences acquire an image differently than 2D sequences?

Back 3

3D sequences use RF energy to disturb the protons in an entire volume of anatomy, and then apply a second phase encoding process to divide the volume of anatomical data into individual slices.

Front 4

In a volume acquisition, frequency encoding is performed along one axis of the imaging volume while phase encoding is performed along the other two. One ___________ is responsible for frequency encoding and the other two ___________ are responsible for phase encoding.

Back 4

Gradients

Front 5

In the ____________ encoding on a 3D sequence, one gradient locates the position of signal along one of the in-plane directions of the image. The other gradient is applied along the slice direction and is responsible for dividing the selected volume into individual slices.

Back 5

Phase

Front 6

The number of phase encoding steps needed along the slice direction is normally equal to the number of:

Back 6

Serial slices desired

Front 7

Signal to noise is proportional to what?

Back 7

The number of protons contributing to the signal of a pixel of an image.

Front 8

In 3D imaging, S/N __________ with each phase encoding step necessary for selecting slices. *In a similar way that S/N __________ that occurs when adding extra signal averages.

Back 8

Increases

Front 9

The more slices and therefore phase encoding steps along the slice selection direction, the ________ the S/N ratio and the _______ the scan time.

Back 9

Greater/ Longer

Front 10

Because the entire 3D volume is selected by RF energy and the slices are selected via __________ encoding, there is no cross talk between adjacent slices within a 3D volume.

Back 10

Phase

Front 11

What are the disadvantages associated within volume imaging that does not occur with 2D imaging?

Back 11

1. Wrap around along the slice direction

Front 12

How do you determine scan time in a 2D sequence?

Back 12

Repetition time x number of signal averages x number of phase encoding steps in the image matrix / the number of echoes contributing to a single rawdata file per TR.

Front 13

How can you determine scan time in a 3D volume?

Back 13

TR x # of signal averages x # of lines in matrix x # of slice / # of echoes contributing to single rawdata file per TR

Front 14

What is different for determining scan time between 2D and 3D imaging?

Back 14

*3D imaging has an additional term for the extra pulse operation required for each slice.
* Scan time of a 2D pulse sequence is INDEPENDENT of the number of slices
* Scan time of a 3D pulse sequence is DIRECTLY PROPORTIONAL to the scan time.

Front 15

What pulse sequence techniques compliment 3D volume imaging?

Back 15

1. Fast spin echo
2. Gradient echo
3. Echo planar imaging

*Conventional spin echo imaging is usually not performed as a 3D technique because of the EXTREMELY long scan times!

Front 16

Can we use 3D volume for any place in the body?

Back 16

Yes, now volume imaging is being performed for all applications, especially when thin slices and high S/N are required.

Front 17

In a 3D volume the effective slice thickness is defined by:

Back 17

The thickness of the 3D volume along the slice direction divided by the number of slices.

Front 18

The smallest volume element in a 3D data set is called a:

Back 18

Voxel

Front 19

The other two dimensions of the voxel are defined by the:

Back 19

In plane resolution of a pixel.

Front 20

How can you calculate the in plane resolution?

Back 20

Divide the FOV by the image matrix along both directions of an image.

Front 21

What are some common MRI post processing techniques?

Back 21

1. Multiplanar reconstruction
2. Surface rendering
3. Volume rendering
4. MIP
5. Substraction

Front 22

______________ ______________ allows images to be created from the volume data in any orientation, after the patient has left. Data is taken from the image data rather than the raw data.

Back 22

Multiplanar Reconstruction

Front 23

Curved MPRs are often used to display the:

Back 23

Coronary arteries *Or other tortorous vessels

Front 24

Would MPRs be diagnostic on 2D images?

Back 24

No, A volume acquisition ensures tht the anatomical data set has a high S/N, thin slices, and no gaps.

Front 25

_________ __________ is another post-processing technique that can be applied to MR images. Is used on 3D images directly ON the image. *No raw data. Produces images of the surfaces of structures which tend to look like black and white photos of actual anatomical specimens. Can be used for surgical planning.

Back 25

Surface Rendering

Front 26

What is the difference between surface rendering and MPR?

Back 26

Surface rendering DOES NOT generate cross sections through the anatomy.

Front 27

What is surface rendering reconstruction process dependent on?

Back 27

Being able to differentiate different tissue types in the volume images.

*This can be done by segmenting or drawing a line around the tissue and then another program will separate the types of tissue through a threshold setting.
*The threshold works by selecting a certain range of pixel values based on the contrast of the tissues.

Front 28

Another method of 3D imaging that incorporates contrast from underlying structures as well as surfaces is called:

Back 28

Volume Rendering

Front 29

In volume rendering each voxel in the anatomical data set is assigned an opacity value as a function of its voxel value based on what?

Back 29

The contrast of the voxel

Front 30

What is a difference between surface rendering and volume rendering?

Back 30

Surface rendering reduces the original data to a surface model and volume rendering potentially incorporates all the data contained in the volume into the displayed image.

Front 31

This post processing technique is typically applied to the BASE images of MR angiography and allows the vessels to be visualized as life like 3 D structures from multiple perspectives by projecting the brightest pixels from vascular structures onto a plane to generate an image of the vessel of interest.

Back 31

MIP (Maximum intensity projection)

Front 32

What is a nice feature of the MIP process?

Back 32

The final image can be spun in numerous directions by the technologist to gain different perspectives.

Front 33

This post processing technique refers to when one set of images from the corresponding pixels on an identically positioned set of images *assuming there is no patient motion, allows some characteristics of the images to be eliminated.

Back 33

Subtraction

Front 34

When would you use the subtraction technique?

Back 34

During contrast enhanced MR angiography

*One set of images are acquired before contrast and another identical scan is done with contrast. At the end the non contrast images are subtracted from the contrast images. At the location where the contrast is not absorbed the non contrast pixel is to the post contrast pixel and following the subtraction all signal is eliminated from the pixel. Where contrast accumulates like in the vessels, when the non contrast pixels are subtracted from the contrast enhanced pixels the bright signal remains.

Front 35

_________ voxels are volume elements with the same dimensions in all three directions.

Back 35

Isotropic

Front 36

__________ voxels are volume elements which are not cubes but rather enlongated or shortened in one or two directions.

Back 36

Ansiotropic

Front 37

To obtain isotropic voxels, what values must be selected for the thickness of the 3D volume?

Back 37

Volume thickness / number of slices = FOV / Image matrix

*Or you can make sure that the effective slice thickness equals the in plane resolution.