CT SIM Basics

Another name for computed tomography

Axial tomography

The plane of an axial image is _________ (perpendicular/parallel) to the long axis of the body

Perpendicular

CT number for accuracy with water is checked on a _____ basis

Daily

CT slices are comparable to…?

A loaf of bread

Z-axis is?

• Length

• Sup - Inf

X-axis is..?

• Width

• Left - Right

Y-axis is…?

• Height

• Ant - Post

T/F: CT vs Therapy, the Y and Z axis are flipped

True

The Gantry has a ___-____ cm aperture

70-90

Smaller apertures are used for ________ (tx/dx) sim, while larger apertures are used for ______ (tx/dx) sim

Dx; Tx

Why are Tx sim apertures bigger

To fit immobilization devices

For dx CT, can you tilt the table?

Yes

Slip Rings allow for what 2 things?

1. Continuous electrical power and communication

2. Continuous gantry movement/rotation

What happens during a helical scan

Gantry moves as the table moves

What does a CT Generator allow for?

Higher power and energy

Do CT Generators have a high or low frequency

High

Where is the x-ray tube in a CT machine

Gantry

Properties of a CT X-ray tube

• Focal spot size

• Rotating or Non-rotating

• Target material

• 0.5-1 mm

• Rotating

• Tungsten

What is the purpose of using Tungsten

To make higher intensity x-rays

What does the Filtration System do

It cleans up lower Energy x-rays that wouldn't be beneficial to the image, making a more uniform beam

Do CT scanners have collimation?

Yes

What controls the slice thickness

Collimation

What is the typical slice thickness

0.5-10 mm

Are the collimators adjustable

Yes

What do the detectors do

Collects info to create the image

What is a common detector type

Scintillation detector

What is it called when you have multiple detectors

Detector array

What shape are the detectors in?

Arc

Is it better to have smaller or larger detectors

Smaller

What are the 2 types of detectors systems

• Single Row

• Multi Row

Single Detector Rows are which generations

3 and 4

What are the limiting factors of single row detectors

• Max slice thickness : 10 mm

• 1 gantry rotation = 1 slice

• Takes a while to get a scan

Multi Row detectors are lined up how?

In parallel rows

Benefits to MultiRow detectors

• Increased scan speed

What is Pitch

Relationship between couch movement and collimation

What can the pitch range from

• 0.5:1

To

• 2:1

Lower pitch has _______ (overlapping/extended) images

Overlapping

Lower pitch results in _______ (lower/higher) patient dose

Higher

Higher pitch = ________ (increase/decrease) scan speed

Increase

When you do a breath hold, do you want a low or a high pitch

High

What are the 2 types of data acquired

1. Raw

2. Image

Raw data is also called ____ data

Scan

How does Raw Data work

• Measurements are taken during the scan by detector array

• Send to computer

• Turned into Image data

What is Image data, basically?

Processed Raw Data

What does a computer do to Image Data?

It assigns a Hounsfield Unit to each pixel

What is a Hounsfield Unit

Shade of Gray

What determines the area in the Gantry where raw data will be acquired?

Scan Field of View

When you have a larger patient, what do you need to do to the SFOV

Extend

How many shades of gray can be seen by the human eye

40

Window width is?

Number of shades of Gray (HU)

What happens when you increase window width? What happens to contrast?

You see more shades of gray

• Decrease contrast

Smaller window width is better which tissues

Tissues of similar densities

• brain

What is Window Level

Center/Median value of the Width

Window Level affects image ______

Brightness

When you think Window Level, think?

Curtain (or blinds)

• you open them and see more brightness from outside

What are Localizer scans

• 2d or 3d

• Initial scout

• 2D

What is a localizer scan used for

Selecting the region of interest (SFOV)

What are Interpikation algorithms used for

Image reconstruction

What is a matrix

Cell layout

• rows vs. columns

Pixel stands for

Picture element

A pixel is a ____D representation of each _____ in a matrix

2; Cell

T/F: Each pixel can be different shades of gray

True

Shades of gray depend on what

Attenuation

What does a pixel measure

Relative Linear Attenuation Coefficient of tissue

Voxel stands for

Volume element

Is a Voxel 2D or 3D?

3D

How do you calculate a Voxel

Pixel x Slice Thickness

Bigger matrix = _____ (less/more) pixels = ____ (worse/better) quality

More; Better

Typical matrix of a CT Scanner

512 × 512

HU are also known as

CT number

What are HU important for?

• Tx planning

• Dose calcs

What do CT SIM machines use to generate BEV DRR’s

Virtual Sim Software

T/F: Wall lasers are at a fixed distance from internal lasers

True

How far are Wall lasers from internal lasers

50 cm

Thinner slices = ______ (lower/higher) quality

Higher

Typical CT Sim Slice thickness and spacing

• 2-3 mm thick

• <5 mm apart

More space between slices means?

More interpolation

What are 2D representations of port films

DRRs

What does mA stand for

Milliamperage

What does mA tell

Beam quantity

What is beam quantity

How many x-rays were made

Increased mA = ______ (inc/Dec) pt dose = _____ (higher/lower) quality image

Increase; Higher

What are key factors for a high quality DRR

1. Slice thickness

2. Spacing

What do virtual Sim softwares do

Outlines

• external contours

• target volumes

• OARs

Shows Beam Placement

Reviews plan and isodose distribution

Why is a virtual sim

Pt is not there when it is done

What is PACS

System that allows for image

• Acquisition

• Interpretation

• Storage

PACS is comparable to what

One Drive

What does DICOM do

Creates standards for image and rad info transfer

DICOM standards are set by who

Joint Committee

DICOM stands for

Digital Imaging and Communications in Medicine

PACS stands for

Picture Archiving and Communication System