Basics of Computerized Axial Tomography

Tomography definition

tomos: “slice”

graphein: “to write”

What are some exams that CT has replaced?

gallbladder tomograms, skull XR, trauma c-spine XR, myelograms, hip fractures, etc.

Why is CT good for evaluating skull and brain?

can differentiate white/gray matter, CSF, clots, bleeding, edema, and neoplasm

What are some drawbacks of CT?

more radiation and more expensive

A CXR gives a dose of ___ mSv; a chest CT gives a dose of ___ mSv

CXR: 0.1 mSv

CT: 7 mSv

What is used instead of shielding in CT?

“beam” collimation

Explain how CT works in comparison with MRI

CT

• better visualization of bony detail, fresh brain bleeding, chest and abdomen imaging

• fast (can be completed in one breath hold)

• uses radiation and detectors

• generally scanned in axial plane

• contra-indications: pt weight

MRI

• better visualization of soft tissue, cartilage, nerves, tendons, brain and spinal cord

• magnetic field instead of radiation

• smaller opening (more confining) and louder

• contraindications: pacemakers, aneurysm clips, metal in eyes, claustrophobia, inability to hold still

What are the benefits of “slicing” in CT?

• creates sections of anatomy

• enables visualization of superimposed anatomical structures

• images can be manipulated after the exam without additional exposures

What is the big “donut” part of the CT scanner called?

Gantry

Briefly explain the 7 different scan generations

1. scans head only with single ray pencil beam

2. single projection straight fan shaped beam

3. single projection fan shaped beam

4. single projection fan shaped beam, stationary detectors

5. electron beam cardiac imaging

6. helical/spiral continuous scanning

7. multidetector (MDCT) simultaneous detectors

Explain the 1st generation scanner

• head only

• head surrounded by water bag

• tightly collimated pencil beam

• translate-rotate motion scanning

• about 5 minutes scan time per image

Explain the 2nd generation scanner

• straight fan shaped beam

• up to 30 detectors

• produced increased scatter

• scans head and body

Explain the 3rd generation scanner

• wider fan shaped beam

• curved array of 250-750 detectors

• tube and detector rotate around patient

• prone to ring artifacts

Explain the 4th generation scanner

• single projection fan shaped beam

• 600-2000 stationary detectors arrayed in 360^o ring

• limited dynamic scanning capabilities

• tube rotates around patient

Explain the 5th generation scanner

• Electron Beam CT (EBCT)

  • rotating electron beam

• used to image heart

• electron gun replaces x-ray tube

• real time image of beating heart

Explain the 6th generation scanner

• enabled by slip ring technology

• tube scans continuously around entire body without stopping to unwind cables

• continuation of the 3rd generation scanner

What are some advantages of 6th generation scanners?

• faster

• requires less contrast

• better sharpness of detail because scan completed in 1 breath hold

• eliminates variations due to inspiration

Explain the 7th generation scanner

• improvement from the continuous scanners

• uses thick x-ray beam with multiple parallel detector arrays to collect information from multiple slices

• may also be called MDCT, multi-slice scanners, multi-row detector scanners, MSCT, or multi-array scanners

• built with 3rd generation architecture

What are some advantages of 7th generation scanners?

• increased anatomical coverage (faster scans)

• scanning with thinner slices to improve resolution

• reduced patient exposure

What are the 3 different scanning methods?

1. localizer image

2. conventional

3. helical

Explain the localizer image scanning method

• single projection image used to position the slices at the location we want to collect images

• stationary tube (table is in motion)

• looks like a radiograph

Explain the conventional “serial” scanning method

• tube rotates around patient while the table remains stationary, then table moves forward in increments after each tube rotation

• yields the best image quality

• used for neurological work, calcium scoring studies, and obese patients

Explain the helical scanning method

• tube remains on, rotating around patient while the table is simultaneously fed through the gantry

• faster (more coverage in a breath-hold)

• no missed anatomy

• less contrast needed

• ideal data for post-processing

The operator console is used for ___

accessing RIS, selecting protocols, viewing images, post-processing images, archiving images, and filming images

Technologist is responsible for setting the:

kV, mAs, pitch, SFOV, DFOV, and scan parameters

What does the digital to analog converter do?

converts the digital commands coming from the scan controller into analog commands that can be executed by the mechanical components

List and explain the 2 types of collimation

1. pre-patient collimator: reduces patient exposure and scatter production

2. pre-detector collimator: limits portion of detector exposed (reduces scatter)

For an x-ray photon to generate a signal, it must:

enter the detector, collide with a detector atom, and produce a measurable event

What are the 2 types of detectors?

gas or solid

Explain solid state detectors

• “scintillation detectors”

• detector material is solid crystalline substance

• more sensitive to incoming x-rays at various angles

• type used in modern scanners

What is the process that solid detectors go through?

1. incoming x-ray photon passes through patient’s body and collides with solid state material

2. detector “scintillator” emits a brief flash of light

3. light is converted to electrical impulse (signal) by photodiode

4. signal is amplified and conducted to array processor where it is reconstructed into an image

What controls resolution in a single detector array?

What controls resolution in a MSCT/MDCT?

single detector array: beam width

MSCT/MDCT: detector size

Explain CT pitch

• table speed

• with helical scanning the pitch is how close or stretched out the loops of the helix are

• stretched out helix loops = increased pitch = less rotations around patient = less patient dose

Equation to calculate pitch

P = I/B

P - pitch

I - table movement

B - beam width in mm

Is 2:1 or 1:1 a higher pitch?

2:1

(faster scan with less detail)