CT I

Shanks Redhaired · 7問 · 1年前

通報

問題一覧

tomos

slice/section

graphien/graphia

describing

Creation of cross-sectional tomographic section of the body with a rotating fan beam, a detector array and computed reconstruction.

Computed tomography

Describe the fraction of beam of X-rays that is absorbed or scattered per unit thickness of the absorber

linear attenuation coefficient

Ct images are a display of the amount of what?

attenuation that occured when x-ray beam penetrates the body

Square shade of gray

pixel

Volume of gray

voxel

This is a result of a computer averaging of the attenuation coefficients across a small volume of material. This gives depth information.

voxel

Each voxel is about how much? on a side and is as thick as 2 – 10mm depending on the depth of the scanning x-ray beam.

1mm

Each voxel is about 1mm on a side and is as thick as how much? depending on the depth of the scanning x-ray beam.

2 to 10mm

represent the percent difference between the x-ray attenuation coefficient for a voxel and that of water multiplied by 1000.

CT numbers

The ability to differentiate between different tissue densities in the image

contrast resolution

Ability to see small objects and details that have high density difference compared with background.

high contrast

These contrast have very high density differences from one another.

high contrast

This contrast has ability to see a small, dense lesion in lung tissue and to see objects where bone and soft tissue are adjacent

high contrast

Ability to visualize objects that have very little difference in density from one another.

low contrast

This contrast is better when there is very low noise and for visualizing soft-tissue lesions within the liver

low contrast

Can differentiate gray matter from white matter in the brain.

low contrast

can degrade image quality and affect the perceptibility of detail. Includes

artifacts

Artifacts due to patient motion, metal, noise, mechanical failure.

streaks

Artifacts due to bad detector channels.

ring, bands

Artifacts can occur due to incomplete projections.

shading

Cause of streaks artifact

pt motion, metal, noise, mechanical failure

Cause of ring and bands artifacts

bad detector channel

Cause of shading artifacts

incomplete projection

Principal Principles of CT

data acquisition, processing, image display

Data acquisition principles

attenuation, data acquisition geometry

Data processing principle

ct numbers, ct and energy dependence

Technological consideration

data flow in ct scanner, sequence of events

Limitations of radiography

superimposition, quantitative

Limitations of ct

persistent image blurring, degradation of contrast

Goal of ct

minimal superimposition, improve contrast, record very small differences in tissue contrast

Systematic collection of information from the patient to produce the CT image.

data acquisition

Two methods of data acquisition:

Slice by slice, volume

Reduction in the intensity of beam of radiation as it passes through an object.

attenuation

Attenuation depends of

Tissue density, electron per gram, atomic number, radiation energy

The way the x-ray tube and detectors are arranged to collect transmission or penetration measurements.

data acquisition geometry

Constitutes the mathematical principles involved in CT

data processing

In CT, a high kVp technique (about how many? kVp) is generally used

120

Image display

display device, windowing, format of the ct image

Sequence of Events

data flow in ct scanner

Pioneers of ct scan

Godfrey Hounsfield, Dr. James Ambrose, Allan Mcleod Cormack, Dr. Robert Ledley, Dr. Willi Kalender

Two elements in the basic scheme in the Data Acquisition

beam geometry, component comprising the scheme

DATA ACQUISITION GEOMETRIES

parallel beam, fan beam, spiral geometry

First generation motion

translate translate

Second generation motion

fan beam translate rotate

Third generation motion

wider fan beam translate rotate

Fourth generation motion

Wider fan beam rotate stationary

Fifth generation motion

stationary stationary

How many detectors in first generation

Beam geometry in 1st generation

parallel beam

How many rays in first generation

160 across a 24 cm fov

In first generation how many degree(s) interval per rotation

How many projections in 1st generation

180

1st generation scan time

4.5 minutes

1st generation reconstruction time

1.5 minutes

What detector signal decayed slowly, affecting measurements made temporally too close together

sodium iodide

geometry allowed very efficient scatter reduction, best of all scanner generations

pencil beam

How many detectors in 2nd generation

How many rays in 2nd generation and views in that ray

600, 540

Shortest scan time of second generation

18 secs

Beam geometry used in 2nd generation

narrow fan beam

Beam allows more scattered radiation to be detected

narrown fan beam

How many detectors in 3rd generation

800

Mechanically joined x-ray tube and detector array rotate together

3rd gen

Angle of fan beam increased to cover entire patient

3rd gen

Eliminated need for translational motion

3rd gen

Newer systems have scan times?

½ second

Designed Designed to overcome the problem of ring artifact

4th gen

How many detectors in 4th generation

4800

Developed specifically for cardiac tomographic imaging

5th gen

No conventional x-ray tube; large arc of tungsten encircles patient and lies directly opposite to the detector ring

5th gen

Electron beam steered around the patient to strike the what? target

annular tungsten

5th gen scaan time

50 msec

Capable of 50-msec scan times; can produce fast-frame-rate CT movies of the beating heart

5th gen

Helical CT scanners acquire data while the table is moving

6th gen

What cT scanners acquire data while the table is moving 6th gen

helical

Allows the use of less contrast agent and increases patient throughput

6th gen

In some instances the entire scan be done within a single breath-hold of the patient

6th gen

When using multiple detector arrays, the collimator spacing is wider and more of the x-rays that are produced by the tube are used in producing image data

7th gen

requires a scout radiograph for anatomical orientation and scan region (slice) selection and scan performed in sequential or helical mode.

routine scan

similar to a conventional radiograph, is very useful for selection of single slices or complete scan regions.

scout radiograph

Scout radiograph other names

survey rad, localizer rad, scanogram, topogram, scout view

This radiograph is taken with a low dose and low spatial resolution by transporting the patient slowly through the field of measurement with the X - ray tube in a fixed position with radiation emitted continuously or in pulsed mode.

scout radiograph

are particularly useful to select the gantry tilt according to anatomy.

lateral scanogram

For a long time, CT examinations consisted of this scan mode

sequential scanning

Sequential scanning other names

axial scanning, single slice scanning

It is scanned, then the patient is transported for a scan increment, mostly equal to the chosen slice thickness. Then, a second scan is taken and the procedure is repeated

sequential scanning

This examination mode is relatively time - consuming and has been largely replaced by the faster helical CT.

sequential scanning

One fundamental disadvantage is that overlapping images for 3D image reconstruction are generally not available

sequential scanning

offer automated and therefore fast modes for scanning single slices sequentially. Cardiac scanning may be a future indication.

modern scanners

Modern scanner offer automated and therefore fast modes for scanning single slices sequentially. WHAT? scanning may be a future indication.

cardiac

is used to record temporal changes in the density characteristics of an object. Typically, dynamic scanning is used to assess contrast medium dynamics

dynamic ct

A representative selected slice is scanned repeatedly or multiphase examination of a complete organ is performed before, during and/or after administration of contrast medium

dynamic scanning

Dynamic scanning other name

serial scanning

The observed changes may represent physiological processes, such as heart motion or breathing, or pathological processes such as portosystemic shunts.

dynamic scanning

is a minimally invasive technique, which provides an excellent 3D representation of portal and hepatic vascular anatomy.

dual phase ct angiography

methods serve to obtain information about the material composition in the tissues examined.

material selective scanning

Material selective scanning other name

dual energy ct

100

To achieve this, a selected slice is scanned with two different spectra, i.e. with different high - voltage values and possibly with different filtration.

material selective scanning

projection

projection

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