Course 111 - Diagnostic Imaging II - 6.1, 6.2 - CT Systems and Mammography

CT components

• Gantry

• Operator’s console

• Table

• Computer

• Multi-format laser camera

• Viewing station

Gantry includes:

• X-ray tube

• Collimators

• Detectors

• High voltage generator

Types of gantries:

• Moving

• Stationary

• Combination

Moving Gantry

X-ray tube mounted on circular gantry assembly that rotates. Connected with either cables with sliding electrical contacts or slip rings

Pre-patient collimation in moving gantry

Located on the tube head, it determines the angular span and thickness of beam

Post-patient collimation in moving gantry

Located on the detectors, it reduces scatter radiation and improves image contrast

Post-patient collimation help to filter X-ray beams for:

• Beam hardening

• Compensation

Beam hardening (Anode-Heel Effect)

In CT imaging the X-ray beam creates image artifacts because the peripheral tissue is exposed to lower average photon energy than the inner portion of the slice

Compensation (Bow-Tie filter)

Unit compensates for the non-uniform thickness of the human body as the edges are thicker

Stationary Gantry

Electron Beam Tomography (EBT)

Contains no moving part that uses a large arc of tungsten that encircles the patient, which allows scans to be done in less than 50 ms

Combination Gantry

Combines nuclear medicine and Computed Tomography

Multi-format laser is typically either…

Dry chemical image or conventional laser film

CT image formation has three distinct phases:

• Scanning phase

• Reconstruction phase

• Analog to digital conversion phase

Scanning phase in CT image formation

X-ray beam is scanned around the body

Reconstruction phase in CT image formation

Uses Filtered Back Projection to build up the CT image in the computer by reversing the acquisition steps. Areas of high attenuation reinforce each other while areas of low attenuation reinforce each other.

Calculating the CT number occurs in this phase

CT number (Hounsfield number [HU])

Represents tissue volume, it’s related to the attenuation of tissues detected. Denser = higher CT number (lighter)

What is the reference material for CT numbers assigned with the value 0?

Water

Analog to digital conversion phase in CT imaging formation

As thousands of analog signals are captured by the detectors, the signals are grouped together in order to place them into a level of gray. The matrix of pixels with each pixels result a digital image that is then converted into a visible image represented by different shades of gray or brightness level on the monitor

Voxel

Volume pixel

Level windowing

The center of the window

Width window

The range of CT numbers that will be displayed with contrast

Reducing window width ___ the displayed image contrast among tissue

Increases

Human eye cannot differentiate different grey levels of less than ___, whereas CT can differentiate of less than ___

10%, 1%

Aperture

Hole in the middle of gantry

How many slice counts could a CT system perform?

4-640

Aperture range of diameters to accommodate patient size

27.5 to 35.4 in

Dynamic volume CT

Scans entire organ in one rotation

Scout/Pilot/Serview-Tomogram

Used to establish where the target organ is located for future treatment or imaging

Axial CT study is also known as..

“Steep and shoot” (translate-rotate) acquisition

What’s ideal for capturing images of the heart and coronary artery calcium, and predicting rick of coronary artery disease?

Electron Beam Tomography (EBT)

Helical or Spiral CT

Provides continuous rotation of gantry while table is also moving. The tube’s constant motion ensures no interscan delay between slicesM

Multi-slice CT

Benefits are increased speed of volume coverage allowing a large anatomical area to be scanned at one time

Advantages of CT machine:

• Eliminates superimposed images outside area of interest

• Can be viewed in axial, coronal, or sagittal plane

• involuntary movements doesn’t cause artifacts

Disadvantages of CT machine:

• Cost

• Amount of radiation

• Allergic reactions to contrast agents

Main safety concerns for Computed Tomography

• Ionizing radiation

• High voltage/current

Widespread use of Cone Beam CT is due to two factors:

• Complex computation capabilities

• Continuous exposure at higher levels required in CT

Pulsed X-ray

X-rays are released in discrete increments

Sievert

Standard international unit for biological harm caused by absorption of one Gray of radiation energy

Four main parts to Cone Beam CT

• X-ray generation

• Image detection system

• Image reconstruction

• Image display

Image detection system includes:

• Rotating gantry

• Fixed x-ray tube

• Fixed detector

in Cone Beam CT image formation, the pulse s of radiation are timed for…

3-4 seconds of radiation exposure

Cone Beam CT is named for it’s shape

X-ray is focused (collimated) at its source and then diverges into a fan shape

The Cone Beam CT X-ray FOV is determined by:

• Detector’s size and shape

• Beam projection geometry

• Ability to collimate the beam

Cone Beam CT uses two different types of detectors:

• Image intensifier tube with charge-coupled device

• Hi-res flat-panel imager

Cone Beam CT’s single rotation, this many layered images are acquired in the FOV and in this duration

150 to 600 images in 10-30 seconds

In Cone Beam CT, to create a viewable picture in less time, it typically uses…

One computer for acquisition and another for viewing

On average, Cone Beam CT reconstruction should take less than __ for standard resolution scans

Three minutes

Stereolithography

3D rendering of the body part scanned

What is a stereolithography used for?

• Plan surgical procedures

• Fabricate custom prostheses

• Model facial-plastic reconstructive procedures

Cone Beam CT is used primarily to…

Image smaller parts of the head and neck

Cone Beam CT common uses

• Dental (teeth)

• Oral/maxillofacial region (mouth, jaw, neck)

• ENT (ear, nose, throat)

Cone Beam CT tube head and detector assemblies are typically mounted on…

Vertical stand that allows up/down movements

Advantages for Cone Beam CT

• Compact

• Faster (less movement artifacts)

• Lower power requirements

• Less ionizing radiation

Disadvantages for Cone Beam CT

• Limited scanning abilities

• Less nuance grey-scale

• Accessories distorts images

• More scatter radiation

Safety concerns for Cone Beam CT

• Ionizing radiation

• Electrical shocks

• Heavy weight

Mammography typically uses Mo-K soft x-rays, operating at ___

24-37 kV

Fibroglandular tissue

Type of breast tissue that’s denser and contains connective tissue, milk ducts and lobules

Cyst

Closed sack of tissue, liquid or gas

Mammography are used solely to…

Image the breast to detect different types of tumors and cysts

Mammography systems can show changes in the breast up to ___ before patient or physician can feel them

2 years

Mammography patients are required to set appointments in…

At least one (or more) scan per year based on age/health history

Mammography units employ…

High-contrast, low dose X-ray

Mammography systems use hi-res __ and hi-def __ systems

Film, detectors

Items that mammography mistakenly scan as calcium spots:

• Deodorant

• Talcum powder

• Lotion

Various ways mammography images can be viewed:

• Hard copy film

• Digital image

• Laser film print outs

Full field Digital Mammography (FFDM)

Process of capturing an image using digital detectors, which displays a soft copy at a digital mammography workstation

In mammography, film is the preferred imaging receptor for…

• Patients with known risk factors and dense breast tissue

• Perimenopasusal

• Under 50 years old

Advantages for FFDM

• Transmitting soft copy electronically

• Instantaneous image acquisition

• Image manipulation

• ¾ of ionizing radiation needed

Computer-Aided Diagnosis is used to scan possible cancerous areas and…

Tags the areas of concern for closer examination by Radiologist

Used to supplement diagnostic mammography:

• Ultrasound

• CT

• MRI

• Nuclear medicine imaging

In mammography, Ultrasound supplement is useful for…

Imaging cysts and guiding breast biopsy

In mammography, CT supplement is useful to…

Determine large cancerous mass has moved into chest wall

In mammography, MRI supplement helps…

Radiologist diagnose and differentiate breast cancers

In mammography, Nuclear medicine imaging supplement is good for…

evaluating spread (metastasis) of cancer into lymphatic system, organs, and skeletal system

Types of mammography exams:

• Screening

• Diagnostic

Mammography paddles are made of…

Plexiglas or other plastics

Connective tissues around the breast ducts are ___ than fat

Denser

Mammography Quality Standards Act (MQSA) requires:

• Annual inspections

• Accreditation every three years through FDA-approved body

Mammography Radiology technologist requirements per MQSA:

• 40 hour training in breast anatomy/physiology, positioning and compression, quality assurance/control techniques, and imaging of patients with breast implants

• 25 supervised examinations minimum

  • +Eight hours of mammography modality training

High Precision Electronic Mammography Compression Scale

Device used to measure pressure applied by the paddle in either pounds or kilograms