l 6 ct physics

INTRODUCTION TO CT PHYSICS

Shahid Younas Lecture 6

Computed Tomography


Which is true for 5th Generation CT Scanner

A. does not use a conventional x-ray tube

B. Multiple array of detectors

C. Bed moves with variable pitch

D. useful for cardiology and nephrology

E. Fast scanning but negligible radiation dose

Computed Tomography


What is the alternative name for the 5th Generation CT

A. CBET

B. Cine CT

C. Cardiac CT

D. Rotate / Rotate CT

E. Stationary / Rotate CT

Computed Tomography


What special reconstruction method is used in Helical CT?

A. Simulation

B. Matriculation

C. Intercalation

D. Interpolation

E. Manipulation

Computed Tomography


Which is the in-correct statement for 6th Generation CT?

A. Volume scanning

B. Short scan time

C. Helical scanner

D. Spiral scanner

E. Inconsistent set of data

Computed Tomography


Which one is in-correct about in Helical Scanning?

A. Pitch = table feed per rotation/collimation

B. The larger the table feed, the faster a body scanned

C. use of less contrast agent

D. increases patient throughput

E. No slip ring technology

Computed Tomography


The 3rd and 4th Generation CT Scanners moves continuously due to circular contact with sliding brushes decreasing the scanning time because of

A. Multiple detector array

B. Non conventional X ray tube

C. Wide Fan Beam Geometry

D. Continuous arc of detectors around patient

E. Slip Ring contacts

Computed Tomography


Which one describes the Seventh generation CT scanner?

A. Rotate / Translate Pencil Beam

B. Rotate / Translate Narrow Fan Beam

C. Rotate / Rotate Wide Fan Beam

D. Rotate / Stationary

E. Multiple Detector Array

Computed Tomography


Seventh generation CT scanners were based on ?

A. Effective usage of x-ray

B. Narrow collimation

C. Narrow Fan Beam

D. Small detector size

E. Low Pitch high contrast

CT Detectors- XENON DETECTOR


CT Detectors

1. Xenon

2. Solid State

3. Multiple detector array



Xenon detector arrays are a series of highly directional

xenon filled ionization chambers.

Current ~ Intensity



Do you remember the detection efficiency of detectors? Which is

the best gas detectors or solid state detectors?

High-pressure xenon detectors provide detection efficiencies of about 50%. The detection efficiency of solid-state detectors

used in CT is about 80%.



Although a gaseous detector does not have the same detection

efficiency as a solid one, the detector can be made very thick

(e.g. 6 cm) to compensate in part for its relatively low density.



The metal septa that separate the individual xenon detectors

can also be made quite thin, and this improves the geometric

efficiency by reducing dead space between detectors..



The long, thin ionization plates of a

xenon detector are highly directional.

For this reason, xenon detectors must

be positioned in a fixed orientation

with respect to the x-ray source.



Xenon detectors are highly directional. Can we use it in Fourth

Generation CT Scanners?

The long, thin ionization plates of a xenon detector are highly

directional. For this reason, xenon detectors must be positioned

in a fixed orientation with respect to the x-ray source.



Therefore, xenon detectors

cannot be used for fourth-

generation scanners, because

those detectors have to

record x-rays as the source

moves over a very wide

angle.



Xenon detector technology has

been surpassed by solid-state

detectors, and its use is now

relegated to inexpensive CT

scanners.

CT Detectors


Solid State Detectors

CT Detectors- Solid State Detectors


Scintillator material

Photo-detector

scintillator visible light

light intensity into

electrical



CdWO4

Yttrium

Gadolinium ceramics

Do you know which solid state detectors are used in present day

CT Scanners?



Because of density and effective atomic number solid-state

detectors typically have better x-ray absorption efficiency.

However, to reduce crosstalk between adjacent detector

elements, a small gap between detector elements is necessary,

and this reduces the geometric efficiency somewhat.



The detector size in CT is

1.0 x 15 mm or 1.0 x 1.5

mm for multiple detector

array scanners


The top surface of a solid-

state CT detector is

essentially flat and therefore

is capable of x-ray detection

over a wide range of angles,

unlike the xenon detector.


Solid-state detectors are

required for fourth-

generation CT scanners,

and they are used in most

high-tier third-generation

scanners as well.

CT Detectors- Multiple Detector Array


The multiple detector

array is an assembly of

multiple solid-state

detector array modules.


With a traditional single detector

array CT system, the detectors

are quite wide (~15 mm) and the

adjustable collimator determines

slice thickness.


Slice width is determined by

the detectors not by the

collimator (collimator does

limit the beam to the total

slice thickness).


slice width is determined by

grouping one or more detector units

together.

The individual detector elements are

1.25 mm wide, and there are 16

contiguous detectors across the

module.


For a third-generation multiple

detector array with 16

detectors in the slice thickness

dimension and 750 detectors

along each array, 12,000

individual detector elements

are used.


The detector dimensions are

referenced to the scanner's

isocenter - the point at the

center of gantry rotation.


All currently planned multiple

detector array scanners make

use of third-generation

geometry to reduce the number

of detector hence cost.

Detail of Acquisition

Slice Thickness: Single Detector Array Scanners


The slice thickness in single detector array CT systems is

determined by the physical collimation of the incident x-

ray beam with two lead jaws.

As the gap between the two lead jaws widens, the slice

thickness increases.



The width of the detectors in the single detector array places an

upper limit on slice thickness.

Opening the collimation beyond this point would do nothing to

increase slice thickness, but would increase both the dose to the

patient and the amount of scattered radiation.



Increased slice thickness increases the number of x ray detected

BUT

Signal to noise ration increase linearly



Larger slice thicknesses yield better contrast resolution (higher

SNR) with the same x-ray techniques,

but

the spatial resolution in the slice thickness dimension is reduced.



Thin slices improve spatial resolution in the thickness

dimension and reduce partial volume averaging.

For thin slices, the mAs of the study protocol usually is

increased to partially compensate for the loss of x-ray photons

resulting from the collimation.

Self Explanatory!

l 6 ct physics

Healthcare

rotate ct

helical ct

cine ct

cardiac ct

individual xenon detectors

highpressure xenon detectors

generation ct scanners

xenon detector arrays