What is the physics in CAT scan?

3d 360 scanner image

sammy

particularly electricity and magnetism (the one who can answer this is really smart) hehehe

Answer
X-ray slice data is generated using an X-ray source that rotates around the object; X-ray sensors are positioned on the opposite side of the circle from the X-ray source. The earliest sensors were scintillation detectors, with photomultiplier tubes excited by (typically) cesium iodide crystals. Cesium iodide was replaced during the eighties by ion chambers containing high pressure Xenon gas. These systems were in turn replaced by scintillation systems based on photo diodes instead of photomultipliers and modern scintillation materials with more desirable characteristics. Many data scans are progressively taken as the object is gradually passed through the gantry. They are combined together by the mathematical procedures known as tomographic reconstruction. The data are arranged in a matrix in memory, and each data point is convolved with its neighbours according with a seed algorithm using Fast Fourier Transform techniques. This dramatically increases the resolution of each Voxel (volume element). Then a process known as Back Projection essentially reverses the acquisition geometry and stores the result in another memory array. This data can then be displayed, photographed, or used as input for further processing, such as multi-planar reconstruction.

Newer machines with faster computer systems and newer software strategies can process not only individual cross sections but continuously changing cross sections as the gantry, with the object to be imaged, is slowly and smoothly slid through the X-ray circle. These are called helical or spiral CT machines. Their computer systems integrate the data of the moving individual slices to generate three dimensional volumetric information (3D-CT scan), in turn viewable from multiple different perspectives on attached CT workstation monitors. This type of data acquisition requires enormous processing power, as the data are arriving in a continuous stream and must be processed in real-time.

In conventional CT machines, an X-ray tube and detector are physically rotated behind a circular shroud (see the image above right); in the electron beam tomography (EBT) the tube is far larger and higher power to support the high temporal resolution. The electron beam is deflected in a hollow funnel shaped vacuum chamber. X-rays are generated when the beam hits the stationary target. The detector is also stationary. This arrangement can result in very fast scans, but is extremely expensive.

The data stream representing the varying radiographic intensity sensed at the detectors on the opposite side of the circle during each sweep is then computer processed to calculate cross-sectional estimations of the radiographic density, expressed in Hounsfield units. Sweeps cover 360 or just over 180 degrees in conventional machines, 220 degrees in EBT.


CT scanner with cover removed to show the principle of operationCT is used in medicine as a diagnostic tool and as a guide for interventional procedures. Sometimes contrast materials such as intravenous iodinated contrast are used. This is useful to highlight structures such as blood vessels that otherwise would be difficult to delineate from their surroundings. Using contrast material can also help to obtain functional information about tissues.

Pixels in an image obtained by CT scanning are displayed in terms of relative radiodensity. The pixel itself is displayed according to the mean attenuation of the tissue(s) that it corresponds to on a scale from +3071 (most attenuating) to -1024 (least attenuating) on the Hounsfield scale. Pixel is a two dimensional unit based on the matrix size and the field of view. When the CT slice thickness is also factored in, the unit is known as a Voxel, which is a three dimensional unit. The phenomenon that one part of the detector cannot differentiate between different tissues is called the "Partial Volume Effect". That means that a big amount of cartilage and a thin layer of compact bone can cause the same attenuation in a voxel as hyperdense cartilage alone. Water has an attenuation of 0 Hounsfield units (HU) while air is -1000 HU, cancellous bone is typically +400 HU, cranial bone can reach 2000 HU or more (os temporale) and can cause artifacts. The attenuation of metallic implants depends on atomic number of the element used: Titanium usually has an amount of +1000 HU, iron steel can completely extinguish the X-ray and is therefore responsible for well-known line-artifacts in computed tomograms. Artifacts are caused by abrupt transitions between low- and high-density materials, which results in data values that exceed the dynamic range of the processing electronics.

Computer Freezes!!!!?

Irfan Burn

My computer always hangs when I tried to use any programs that using 3D features. Including hard games, FLASH games, and even simple games like SOLITAIRE. Sometimes, my computer will hang if I played musics and videos.

I often edit videos and I got annoyed because my computer always hang in the middle of converting process.

I've tried to change my RAM, VGA, power supply cable, adding additional fan inside my computer, and using UPS/Stabilizer. But it's no use.

It's fluxtuation was random. Sometime, i was be able to play games a day full. But the next day, my computer hangs every few seconds after I ran 3D programs.

I'm using Intel Pentium 4;
3.06 GHz Dual Core
1,5 GB of RAM (2 slots)
VGA : ASUS Extreme AX550 series
512.0 MB

What should I do...?

Answer
Your configuration of your computer must be wacky.
The common to your problems is the graphic card

go to the manufacturers website and update the driver.

You can look at this it all helps
Also while your computer is in good nick you should download and install superantispyware because when malware gets going you will not be able to access the net.


Clean Up Computer
===============
Download and run superantispyware. I was put onto this program because of an annoying popup (XP or Vista plus all OS preceding)

http://www.superantispyware.com
Superantispyware is a good program and remember to update it before scanning. A scan may take 45mins.

Note Superantispyware has powerful reset tools that will set stuff back to windows defaults.
To use these tools click preferences >repairs. This may be handy to reset IE URLS if you have annoying pop ups.

Also set up tick boxes that you want in the other tabs as you require.

afterwards clean up the registry with this
Clean up pc and IE
==============
To clean up run this Microsoftâs windows live program. It does registry, spyware , virus , disk clean up and defrag

XP Users http://onecare.live.com/site/en-ca/defau...
http://onecare.live.com/site/en-au/default.htm

Vista Users http://onecare.live.com/site/en-ca/cente...

Run the full safety scanner, it takes a few seconds to install the scanning files but when you start the scan, iIt takes 2 hours or more. So once it starts to load x file of x you can leave the computer and go to bed etc.

Then at the end of the scan it asks you to set it running full time.

Think twice about setting it to run in the background as it could be another thing to slow you down. However, it may be worth while if you have no other security programs running.

Consider the following options:
I personally run Norton 360 which is full internet security and run superantispyware when I want to.

So if you donât have a decent internet security program then perhaps run the superantispyware with your antivirus in the background.

If you sick of antivirus programs that donât work perhaps run onecare full time and run superantispyware whenever.

Now you should be flying and Onecare has even set up a new system restore point for you.

Note onecare does other stuff as well. Click on the menus on the LHS column.

add onecare to your favourites

Slow computer IE freezes
==================
Note: only have to do Safe mode if the files will not delete in normal mode. Try normal first.

Slowly tap the F8 key while the computer is powering up

Just after Verifying DMI files occurs the F8 key will point the computer to a menu where safe mode can be selected.

Note if you mistakenly tap F8 key again, it will continue through to normal boot

Scroll and select safe mode with network

Use explorer to delete the contents of the folder datastore.

Datastore can be found in c:\windows\softwaredistribution.

You may have to do this regularly when the computer slows.

The folder rebuilds itself and the contents are to do with windows updates and logs for errors. There is no harm in deleting the contents.

Reboot.

Also
Over time IE7 seems to set its own (or programs do) tick boxes in the advance tab.
In IE7 (IE6) Tools >Internet Options >Advance Tab
Click Restore advance settings defaults
Tick empty temp files when browser closes (this empties the cache)
Bullet disable phishing filter (IE7)
Delete Browser History, cookies the whole lot regularly.
Close the browser to set the settings.

Optional speed fix
=============
If you want IE7 to speed up and work like IE6 then do this (you can undo it later)
Click tools >manage add ons > enable /disable >select each add on and bullet the disable button
Close the browser to set the settings


RAM and Virtual Memory check
========================
Open up the internet to eg yahoo answers and have a few things open .(eg Word excel or a couple browser pages)
If you normally play music, play that.

Open Task manager >performance tab
Look at the RAM usage in the physical memory (k). If you are using more than 60% of your RAM while your computer is only doing the above then there is not much left for video or games. You may need to install more RAM
1 Gig for XP, 2 Gig for Vista

Look at the PF usage. That is the virtual memory .

Your virtual memory should be set to 1.5 times the amount of RAM you have. Eg 1 gig RAM, therefore 1.5 gig Virtual Memory

Right click my computer icon >properties > advance tab >performance settings > advance tab > virtual memory change > select the drive with the operating system on it (normally C:) bullet custom. Enter the min and max value as one and a half times the RAM you have. In this example, 1500 min and 1500 max. >set.

Your computer is now set for optimum performance for the RAM you have.


Also right click on my computer icon >properties > advance tab >performance settings> the default is let windows, you could try best performance.

Powered by Yahoo! Answers

Title Post: What is the physics in CAT scan?
Rating: 100% based on 99998 ratings. 5 user reviews.
Author: Yukie

Thanks For Coming To My Blog