(E1) Data Acquisition/ image recon

what is data acquisition?

method by which a patient is scanned to obtain enough data for image reconstruction

• how the image is taken

what are the two basic elements for data acquisition:

1. beam geometry

2. components-- physical 9devices

what is beam geometry

size, shape (SFOV) and motion of beam and its path

what are some examples of physical devices

• pre-patient collimators

• detectors

• ADC

what are the basic steps of data acquisition system?

1. the tube and detector are always in alignment

2. the tube a detector scan the patient to collect a large # of transmission measurements

3. the beam is shaped by a special filter (bowtie)

4. the beam is collimated to pass through only the slice of interest

5. the beam is attenuated by the patient, passes through the post patient collimator and the detector measures the transmitted photons

6. the detector converts the photons into an electrical signal

7. the signals are converted by the ADC

8. the digital data is sen to the computer for image reconstruction where mathematical equations occur (algorithms)

what are the three methods of data acquisition

1. localizer

2. conventional/serial CT scan

3. helical, spiral, or volumetric CT

what is a localizer scan (scout image)

stationary tube/ patient table and motion

what is the function of localizer scan

• provides an ijmage of superimposed tissues

• large FOV

• allows alignment of cross sectional slices with specific structures

  • select the DFOV

  • save the scout image with lines on to use as a reference to the cross sectional images

what is raw data

data measured in each projection

conventional/ serial CT scan ( xray tube vs. patient)

• x-ray tube rotates around the patient

  • 3rd vs 4th generation scanners

• patient table remains stationary

  • acquiring adjacent slices

helical/ sprial/ volumetric CT scan is

continuous data collection (volume) through multiple projections during continuous patient translation through the gantry

• needed for 3D image reconstruction

what are the requirements to perform helical scans:

• scanner must be of continuous rotation (slip ring)

• contain high heat capacity x-ray tube

• rapid cooling capacity

the process for raw data

• information must be divided into individual cross sectional slices

• mathemical interpolation must be run to divide the different samples from the different planes and raw data from different slices

  • reconstruction to form final images

what are the advantages of helical CT scan

• complete organs may be scanned in one breath hold

• many slices acquired at a time

• less chance of mis-registration with inconsistent breath holding

• smaller amount of contrast needed

• 3D reconstruction

what is multislice detector array (7th gen)

• requires several parallel detector arrays that contain thousands of individual detectors

• requires fast large capacity computer

  • quickly energizing a large detector array results in larger colume imaging

  • increased coverage with decreased amount of time

what is an algorithm for image reconstruction

set of rules or directions for getting specific output from specific input

• in CT, computers use these algorithms to process raw data into recontruction images

what is fourier transform/ transformation

• primary matematical mehtod used in CT for image reconsturction

• takes complex data and arranges it into simpler and more useful forms

what is interpolation

the projecting of raw data between two known values

• used to create a wide variety of sections that are reconstructions of data

  • no additional exposure

what is extrapolation

the projecting of raw data beyond the range of known values

what is retrospective reconstruction

the saving of the patient’s “raw” data from a scan

• allows post-scanning manipulation

• allows re-manipulation of images from the raw data is saved

what is filter back projection

• filter refers to mathematical function

• images from all projections placed together to form one image

  • raw data from each projection added and averaged

  • image produced is not very sharp

• correction algorithm is applied to accentuate the edges of the info in the raw data

  • prior to image reconstruction

what is convolution

process of modifying pixel values by a mathematical formula through a filter function

• process of applying filtration to the data

what is a “mask” in image reconstruction

overlaps acquired data to recontrust the image

• removes star-like blurs from back projection

what is deconvolution

process of returning the pixel values to their original level by the reverse process

what is multiplanar reformation (MPR)

• post porcessing technique performed on image data which produces new slice from a set of CT scans

  • initiated from console or workstation

• utilized for images in planes that would otherwise be difficult or impossible to acquire

  • must be contigous with no gaps (thin slices)

what is the disadvantage from MPR

very time consuming

• independent workstations

advantage of MPR

software that allows CT to show an entire volume in one image - 3D

what is maximum intenisty projection (MIP)

• simplest form of 3D imaging

• reconstruction can be done quickly

• good for showing vasculture

• only uses 10% of the data points to create a 3D image

what is shade surface display (SSD)

• reconstruction process dependent on separatng different tissue types in the scanned images

  • does not generate images of cross-sections of anatomy but images the surface of the anatomical structure

• stack transverse slices to form volume of anatomical data

applications for SSD

• vessel display, soft tissue, other viscera

• relationships between vasculature and viscera

• surface and internal detail of anatomy

• benefits for orthopedic and craniofacial surgery, neurosurgery, and rad therapy

what is shaded volume display (SVD)

• utilizes a 3D semitransparent representation

• this process utilizes all voxels that contribute to the image

  • shows multiple tissues and their relationship to one another