CT: Data Acquisition Methods (2)

ppt 1-5

Scan Field of View (SFOV)

**Circular region within the gantry** from which the transmission measurements are recorded during scanning

Determines how many detector cells are collecting data

T

T/F: There are usually a few fixed choices for SFOV in the system. \[ Small & Large \]

Display Field of View (DFOV)

Circular region that determines how much of the collected data is **used to create an image.**

Localizer Scan

Preliminary step before taking a cross-sectional image.

Image acquired is similar with a conventional radiographic projection (2D)

T

T/F: Tube and gantry remain stationary during a localizer scan.

(Z D EF— MR)

Determines Z axis coverage

Selecting DFOV and image center

Exposure Factors for CT

Identify mispositioning

Serves as a reference image

5 purposes of a localizer scan.

perpendicular

parallel

The images generated using the step- and-shoot method were — to Z-axis and -- to every other slice

Clustering

A term used when more than 1 scan can be taken in a single breath-hold (Step-and-Shoot Scan)

Good

Good/Bad: Spatial Resolution in a Step-and-Shoot image

Contiguous and non-contiguous slices

2 kinds of image acquisition in a step-and-shoot scanning method.

Cine

**Axial** scans set to **repeat scans at the same slice location**.

Interscan Delay

4 Limitations of Step-and Shoot method

Longer examination time; patient is at the table with no data being acquired.

misregistration

4 Limitations of Step-and Shoot method

Omitted anatomy due to inconsistent patient respiration.

Stairstep

4 Limitations of Step-and Shoot method

Inaccurate generation of 3D or reformatted images producing — artifact.

Contrast Issues

4 Limitations of Step-and Shoot method

only a few slices seen with maximum contrast

Detector

Refers to a **single element** or type of detector used in a CT system.

Detector Array

refers to the **collection of detector elements** used in a CT system

Reference Detectors

elements in a detector array that **measure non- attenuated radiation**; this assists with calibration and artifact-reduction

Single Detector Row System (SDRS)

contained many **detector** elements **aligned in a single row**
• Many detectors along the X-axis

• One row in the Z-axis (1D in z-axis)

Beam Collimation

What determines the slice thickness in SDRS?

patient experience

workflow

image with contrast

Development of reduced scan times were made for (3)

Slip rings and MDRS (multiple detector row system)

2 main drivers of scan time reduction

MDRS

are a **two-dimensional arrangement of detector arrays** where many **parallel arrays are aligned in the Z-axis**


• **Multiple slices can be acquired for each rotation** of the x-ray tube and detector

Detector Electronics

responsible for **digitizing the signals from the detectors** **before** they are sent to the computer for **processing.**

Switches

Data acquisition channels **(DAC)** are coupled to the multiple detector array by what?

Switches

Responsible for **grouping the signals** from individual detector elements and sending them to DAC

1 Slice

Grouped signals from the switches go to a corresponding DAC; **1 DAC corresponds to what?**

Rows

simply refers to the **number of detector elements** lined up in the Z-axis

slice

refers to **how many slices may be obtained for each rotation** of the gantry; this is **determined by the number of data acquisition channels**

binning

__**combining**__ **various detector elements electronically** to produce the desired slice thickness required for the examination

Beam width and detector array configuration

In MDRS, how is slice thickness determined?

Fixed

matrix

Uniform

Variations in MDRS (arrangement)

Detector elements **arranged in parallel rows of equal size**

adaptive

non-uniform

hybrid

Variations in MDRS (arrangement)

DEL arranged in parallel rows of variable size

Isotropic

When the **slice thickness is equal to the pixel size**, all dimensions of the voxel are equal→dataset is —.

Good

Good/Bad: Spatial Resolution in an isotropic image.

F

T/F: Scanning time and radiation dose is reduced when acquiring an **isotropic image**

Helical Scan

the x-ray tube, detector and table **are in motion throughout** the entire data acquisition process

• The result is a volume of data instead of separate individual slices.

Slip rings

Allowed the continuous rotation of the x-ray tube and detectors; eliminating long cables.

Reduced interscan delay.

Increased mAs

One limitation of helical scanning is heat due to continuous exposure and —.

Helical Interpolation

The principal technique used to remove blur and slant in a helical image.

360 LI

An algorithm that interpolates a planar slice using data points measured 360 degrees apart.

180 LI

uses data points **closer to planar slice** to be interpolated

• produces a **sharper** **helical image** (however, with increased **noise**)

Slice thickness blooming

The outcome of this selection is not precise as the true size of the slice thickness is often slightly larger than the size selected

caused by:

* interpolation technique
* table speed
* detector width

Pitch

is defined as the **travel distance of the CT table** per 360° rotation of the x-ray tube, **divided by the x-ray beam collimation width**

\
a ratio that compares how much of the table enters the gantry to how wide the beam is in the Z-axis (for each rotation of the x-ray tube)

1

When the amount of table that enters the gantry is equal to the beam width, the pitch is equal to?

increases

**SDRS: Pitch**

• When we increase the amount of table entering the gantry but keep the slice thickness the same, the pitch

decreases

**SDRS: Pitch**

When we decreases the amount of table entering the gantry but keep the slice thickness the same, the pitch—.

high pitch

high/low pitch: The spiral is extended and stretched and the slant present in the helical image increases

Detector element

The smallest slice width available is determined by