Fluoroscopy + CCDs

carter ch. 6 + bushong 19&20

CCD’s + sequence

• Oldest indirect conversion

• SEQUENCE:

  • X-ray interact with scintillation material

  • Sent to capacitors which convert light into electrical charge

  • Charge sent to ADC

• Structure - silicon chip

• Cesium Iodide-structured phosphor (similar to a TFT) reducing light spread

• how its coupled→Lenses or fiber optics focus light onto chip - beam splitting mirror

• Used in fluoro, C-arms for trauma or forensic and stereotactic breast biopsy

CMOS

Complementary metal oxide semiconductor

• Highly efficient and inexpensive

• More susceptible to noise so lower quality, lower resolution and lower sensitivity as compared to CCD’s

• Convert light into electrons-stored in capacitors within the pixel then to ADC

Binning

Just remember → Binning allows charges from adjacent pixels to be combined on the sensor before the charge is readout through the amplifier, the dominant noise source on a CCD.

-Faster way to get a readout (grocery example = stuffing groceries in 2 bags when it actually 3 bags worth of food)

-combine pixels together to improve image quality

-Part of CCD

Flat panel Fluoro

HISTORY

• Thomas Edison, 1896
• Screen placed over patient’s body in x-ray
beam
• Radiologist looked directly at screen
• Red goggles-30 minutes before exam - night vision
• 1950 image intensifiers developed - intensifying the light of the image

Cones

Central

• Less sensitive to low light (threshold of 100 lux)

• Will respond to bright light

• Daylight vision (phototopic vision)

• Perceive color, differences in brightness

• Perceive fine detail

WE WANT CONES

Rods

Periphery

• Sensitive to low light

• Used in night vision (scotopic vision)

• Dims objects seen better peripherally

• Color blind

• Do not perceive detail

Dont want to use rods, want to use cones

Fluoro xray tubes

• Under the table

• Operate at less than 5 mA, so low because the long exposure time w/ higher kvp so mA has to be lower for correct mass

• KVP dependent on body section

• ABC (Automatic brightness control) = ABS (Automatic brightness stabilizer) = AGC (Automatic gain control) → Maintaining the brightness by changing technical factors automatically

• AERC - Automatic exposure radiation control (using an AEC-controls the time)

Fixed xray tube - How far from the patient?

No closer than 15 inches or 38 cm from the patient, under the table

Carm Mobile - How far from the patient?

May be brought no closer than 12 inches or 30 cm from patient

Which has less radiation Over-couch or under couch?

Under the couch

The Sequence

1. Beam exits the patient - remnent

2. Hits the input phosphore(cesium iodide CsI tightly packed needles… produce excellent spatial resolution)

3. Converts x-rays to visible light

4. Hits photocathode (Cesium and antimony components)

5. Emits electrons when struck by light (photoemission)

6. The potential difference within the image intensifier tube is a constant 25,000 volts

7. Electrons are accelerated to anode → Anode is a circular plate with hole for electrons to go through → Focusing lenses=electrostatic lenses

8. Hits output phosphor which interact with electrons and produce light

9. Electrons hit output phosphor (zinc cadmium sulfide) with high kinetic energy producing an increased amount of light

The electron path

• Must be focused for accurate image pattern → use electrostatic lenses (focusing devices)

• Accelerate and focus electron beam

Label

1. Output phosphor

2. Anode

3. Focal point

4. Electrostatic lenses

5. electrons

6. Photocathode

7. Glass envelope

8. Input Phosphor

What happens after light is emitted from output?

Old school → Was transmitted as an analog signal via a TV tube called Plumbicon/vidicon

Now→ light is captured by a CCD or a FLAT PANEL SYSTEM is
used

Flux gain

1 xray = ? light photons

Comparing the # of x-rays coming in and the # light photons coming out

1 xray = 3000 light photons

-Want a bright image = larger diameter

-smaller diameter = lower brightness → direct relationship

# of output light photon / # of input x-ray photons

Minification gain

Ratio of the square of the diameter of the input phosphor to the square of the diameter of the output phosphor OR

Comparing a change of activated diameter of the input phosphor to the fixed diameter of the output phosphor

-Larger diameter = more minification gain

-Minification goes down = brightness gain goes down

Brightness gain

Flux gain x minification gain

-Ratio of the intensity of the illumination ot the output phosphor to the radiation intensity at the input phosphor

-Brightness gain of 5000-30,000

-Maintaining (automatic) of the brightness is called ABC or ABS or AGC (control,stabilization gain control) or AERC-Automatic Exposure Radiation control- Adjust MA and make more patient exposure

Conversion factor

Ratio of intensity of illumination at the output phosphor (measured in Candela per meter squared) to the radiation intensity at the input phosphor (mGya per sec)
(Cd/m² ) / (mGya/s)

→ miligray coming in and candela coming out

MULTIFIELD IMAGE INTENSIFICATION

Allows focal point change to reduce field of view and magnify the image

-Multifield is different diameters

-Standard component on most machines
-Always built in in digital units
-Most popular is 25/17/12 → 12 on input phosphor = has the least flux gain

-Trifield tubes are 25/17/12 or 23/15/10

At 25 - all photoelectrons are accelerated to output phosphor

MULTIFIELD IMAGE INTENSIFICATION

1. 25 diameter - all photoelectrons are accelerated to output phosphor

2. 17 diameter

3. 12 diameter - more magnification

1. Collimation

2. Capture an image

3. Fluoro

4. Tilting table

5. Moving table

If the diameter is reduced to magify the image- how does that impact flux, minification and ultimately brightness? Patient exposure?

Flux → down

Minification → down

Brightness → down

Patient exposure → up

ABC/ABS/AGC/AERC → increases the mAs

Facts about digital fluoro

• Image acquisition is faster
• Can post process
• Similar equipment to a conventional fluoro room except
• two monitors
• Operates in radiographic mode (400 mA station = better for patient dose)

the x-ray beam is pulsed progressive fluoroscopy

PULSED PROGRESSIVE FLUOROSCOPY

• Generator can be switched on and off rapidly = make it not too hot

• Interrogation time

  • Tube switched on and meets selected levels of kVp and mA

• Extinction time

  • Time required for the tube to be switched off

  • Duty Cycle - time tube is energized

• Each must have times of less than one 1 ms.

FPIR - Flat panel Image receptor

• Radiographic mode = regular MA

• Replacing CCD’s

• Made of cesium Iodide pixel detectors

• Lighter, smaller than image intensifiers

• Improvement to image as the spatial resolution is uniform and distortion free

• High DQE

• Improved contrast

• Rectangular image

Greater density formula

mAs x KVP² / SID² x grid conversion factor

Grid conversion factors

16:1 → 6

12:1 → 5

10:1 → 5

8:1 → 4

6:1 → 3

5:1 → 2

no grid → 1

Advantages of CCDs for medical imagining

High spatial resolution

High SNR

High DQE

No warm up required

No spatial distortion

No maintence

Unlimited life

Unaffected by magnetic fields

Linear response

Lower patient radiation dose

Advantages of Flat planel IR

Distorion free images

Constant image quality

Improved contrast resolution over the entire image

High DQE

Rectangular image

Unaffected by external magnetic fields