Digital Imaging CT & MR

Transcription

1 Digital Imaging CT & MR January 22, 2008 Digital Radiography, CT and MRI generate images in a digital format What is a Digital Image? A digital image is made up of picture elements, pixels row by column matrix Each pixel has a color or shade of grey Rows x Columns = Matrix Size More pixels/area = greater detail More pixels = bigger file size

2 300 Pixels Per Inch 150 Pixels Per Inch Pixels 4x larger A digital radiograph is composed of pixels, each with an assigned shade of gray Pixels must be small, considering small size of some lesions Advantages of Having Digital Radiography Better image contrast easier to see some lesions Very forgiving of radiographic technique settings less retakes due to exposure You can adjust blackness and contrast after the fact post processing Clients will be impressed they will ultimately expect this You can transmit images electronically for consultation More convenient storage No more lost images assuming backup

3 Digital Radiography X-ray exposure is measured on an electronic medium and converted to a digital file in DICOM format Standard x-ray machines are used Digital Image Files The DICOM file Digital Imaging Communications in Medicine DICOM is a file format, like.tif and.jpg Must have a DICOM viewing software to view DICOM files are large One page text document 20k Digital photo 500k Digital radiograph (DICOM) 10000k Critical that your images are really DICOM compliant Have this documented If not, portability is limited Computed Radiography (CR) $60k-$100K Cassettes appear similar to film-screen Cassettes must be processed in plate reader; 90 sec per plate Images better than film good as DR?? Most labor intensive digital system

4 Film CR CR DR CCD Camera Systems $50-100K Image quality is function of CCD camera lens and number of pixels in camera chip lens quality critical CCD Camera Photographs Screen X-ray beam Patient Intensifying Screen Light Mirror

5 Digital Radiography (DR) $100k+ No cassettes, camera or lens Immediately after exposure, image is viewable and ready to archive Excellent quality image Image plate is portable but subject to damage Misconception Inherent detail in a digital radiograph is greater than in a film radiograph Inherent detail is not as good It is the contrast resolution that appears to give digital radiographs more detail Film Digital

6 Film Digital Misconception Radiographic exposure factors are lower with digital systems than with film-screen systems Exposure factors are at least at high, and usually higher, with digital systems Creates radiation safety concerns for staff Misconception Having digital radiography will eliminate artifacts and solve other technical problems Common problems Poor positioning Improper exposure Poor darkroom technique Poor radiation safety practices Insufficient views Wrong part radiographed

7 PACS System Picture Archive and Communication System A centralized computer storage system Storage Dissemination Link image with demographic data CT and MR Imaging Have all advantages of digital radiography Advantages over radiography Better contrast resolution Tomographic MR provides for physiologic imaging since are primarily looking at water CT and MR images are digital images, made up of pixels. Note close up of area in CT image of nasal cavity outlined by white box.

8 Detail vs. Contrast MODALITY DETAIL SPATIAL RESOLUTION CONTRAST RESOLUTION Radiography CT Great Good 0.1 mm 0.4 mm Poor Good MRI OK 1.0 mm Great Contrast resolution trumps spatial resolution CT Contrast Resolution MR Contrast Resolution

9 Tomographic Images Images of slices Eliminates anatomic superimposition Tomographic modalities CT MRI US Tooth fracture Horse with draining tract marked with metal marker Enlarged pulp cavity MRN Fistula How Does CT Work? Is based on quantification of x- ray absorption Similar to radiography, but in slices Image is generated by computer after measuring x-ray attenuation.

10 Generic Scanner An x-ray beam is used to image slices of the patient The thickness of the slice can be varied; 1-10 mm Each slice ~1 sec Table moves after each slice Modern Scanners Table moves continuously Acquire >1 image slice per tube revolution 4,16, 64 slices per rotation Scan patient in a few seconds Provides high quality data for exquisite images Dog had complex skull fractures seen in radiographs that were difficult to assess in transverse CT images. High quality data allowed valuable 3D image to be made

11 Contrast medium can be used with CT to look for BBB leaks, vascular leaks, and hypervascularity Splenic nodule seen after contrast medium given Pre-contrast Post-contrast CT contrast medium is iodinated and thus increases x-ray absorption MRI Contrast resolution much greater than CT Image appearance dependent upon chemical composition Water, free or tissue Fat No ionizing radiation Image in any plane No reformatting CT MR Caudal fossa abscess much more conspicuous in MR image in this patient The physics is much too complex for this introductory class

12 Some MR Facts It s all about magnetism Very strong magnetism Common strength = 1.5 Tesla 1.5 Tesla = 15,000 Gauss Refrigerator Magnet = 50 Gauss Earth s magnetic field = 0.5 Gauss Strong magnetism creates safety concern

13 MR Facts Magnet causes protons to align with field Radio wave (RF) knocks protons out of alignment into X-Y plane Misaligned protons create signal in receiving coil Signal strength converted to grey scale in image High signal is white Low signal is black Steady State RF signal applied to flip vector into X-Y plane Magnet reads signal in X-Y Plane MR Facts We are primarily imaging hydrogen protons, i.e. water Water creates a high signal in some MR image types Most lesions have increased water So, we commonly look for signs of increased water

14 MR Image Names vs. Signal Names relate to physical processes and magnet knobology that determine which tissue types are rotating in the X-Y plane we will not cover so, you just have to remember this stuff Name Fat Free Water Tissue Water Bone T1 White Black Grey Black T2 White White Grey Black Fat Bone CSF Tumor T1 T2 NC12832 Contrast medium can be used in T1 images to look for areas of BBB breakdown, vascular leak and hypervascularity T2 T1 T1+C Same dog as last slide; tumor is more conspicuous with contrast medium MR contrast medium is magnetic and influences proton behavior