W2 - Data Acquisition Methods

Three methods of acquiring data (Types of Scans)

• Localizer Scans

• Sequential Scans

• Helical Scans

Localizer (topograms, scouts)

• Typically, more than one is taken

• Moving table with a stationary tube, not cross-sectional

Sequential scans (axial, step-and-shoot)

Stationary table during scanning, then the tube activates alternately with table movement

Feature of Sequential Scans

Grouping scans:  grouping more than one scan in a single breath-hold (dependent on the speed of the scanner and how long the patient can hold their breath)

Types of Data Acquisition of Sequential Scans

• Continuous = one slice abuts the next

• Gapped = taken when a survey of an area is needed, and imaging of every part of the region is not required 

• Overlapping = rare, because they raise patient dose and don’t provide additional diagnostic info

Benefit vs. Disadvantage of Sequential Scans

• Benefit = very slightly improved spatial resolution due to perpendicular slices (not helical), and the patient is stationary 

• Disadvantage

  • excessive scan time (significant when using contrast media, breathing instructions)

  • inability to create MPR's from raw data (must be helical)

  • Slice misregistration (when a patient breathes differently with each data acquisition)

  • data gaps

Helical Scans (spiral)

• The most common method

• Requires continuous tube/detector activation and rotation while the table feeds through the gantry at a prescribed speed

Volume of data / Feature of Helical Scans

= the result of helical data acquisition (an unsliced loaf of bread)

• Allows the user to virtually reconstruct the data in any configuration (can overlap images without the additional patient dose)

• Can also decrease the partial volume effect by changing the slice center 

Pitch - Helical Scans

= Table movement per rotation/ beam width (or detector collimation)

= Describes the table movement throughout a helical scan acquisition

• pitch<1 = anatomy scanned multiple times (tabled slower)

• pitch>1 = data gaps are created (table faster)

• Pitch = 1 = table feed and beam collimations are identical

Most Common Pitch

1-1.5

Scan Planning

• Patient Orientation = head or feet first

• Patient Position = supine, prone, lateral decubitus

• Centering = centering the pt in the x and y axes

• Zeroing = centering the pt in the z axis

Types of MDCT Detector Arrays

• Fixed

• Adapted

Fixed MDCT

• contain multiple rows of detectors of the same dimension

• It is common practice to combine detector rows during acquisition to optimize data collection

Adaptive MDCT

• allow for a wider variety of detector configurations

• contain detector rows of varying dimensions (in the z-axis)

MDCT Advantage

= ability to acquire multiple/longer/faster discrete slices of data with each gantry rotation

• however, a wider beam decreases IQ; hence, maximum collimation is limited in areas that require high IQ

• only when speed is prioritized, the full detector array will be activated 

Isotropic Data Acquisition

= acquiring helical data in tiny isotropic volumes or voxels

• Allows data to be reconstructed in any plane with equivalent image quality, because the x, y and z dimensions are all equal 

• Accompanied by the thinnest detector rows 

Advanced Data Acquisition Methods

• Dual Source CT

• Dual Energy CT

• Dynamic CT

Dual Source/ Focal Spot CT

= incorporates two sets of tube-detector arrays within the gantry

Pro and Con of Dual Source CT

• Pro = increased scan speeds/ temporal resolution, high-powered scanning for bariatrics

• Con = practically, the second detector array is restricted by the size available in the gantry. Therefore, one detector array covers the entire scan field of view, whereas the second detector is limited to a smaller, central field of view

Dual Energy CT (Spectral Imaging)

= done by operating the tube(s) at two different energies 

Function of the Dual Energy CT

• allows for tissue differentiation through software analysis

• useful when the identification of a particular substance in the body benefits diagnosis

Dynamic CT

= continuous scanning of the same region by continuously scanning with the table stationary

Function/Benefit of Dynamic CT

• Common for blood flow evaluation, aka perfusion study 

• Brain perfusion scans which are performed to evaluate the extent of brain tissue involvement during/following ischemic stroke events.

Detector Binning

Grouping detector rows together in various combinations to acquire the desired number of slices at the desired slice thickness