W2 CT PHYSICAL PRINCIPLES II.pptx

PRINCIPLES OF CT II W2 ILO’S ON COMPLETION OF THIS SECTION, THE STUDENT SHOULD BE ABLE TO KNOW THE FOLLOWING: CT IMAGES DISPLAY HOUNSFIELD UNIT...

PRINCIPLES OF CT II W2 ILO’S ON COMPLETION OF THIS SECTION, THE STUDENT SHOULD BE ABLE TO KNOW THE FOLLOWING: CT IMAGES DISPLAY HOUNSFIELD UNIT ATTENUATION COEFFICIENTS AND CT NUMBERS FOR COMMON TISSUES S. Jastaniah, J. Al-Ghamdi, A. Aref (KAU), A. Musa (KKU), K. Hassan (BMC) 05/31/2024 1  ?WHAT IS DISPLAYED IN CT IMAGES  EACH PIXEL IN RECONSTRUCTED IMAGE IS ASSIGNED A NUMBER THAT ARE RELATED TO THE LINEAR ATTENUATION COEFFICIENTS (Μ) OF THE TISSUES.  THESE ARE CT NUMBERS OR HOUNSFIELD UNITS, NAMED FOR THE INVENTOR OF CT TECHNOLOGY , SIR, GODFREY HOUNSFIELD.  THE MATHEMATICAL RELATIONSHIP BETWEEN LINEAR S. Jastaniah, J. Al-Ghamdi, A. Aref (KAU), A. Musa (KKU), K. Hassan (BMC) 05/31/2024 2 ATTENUATION COEFFICIENTS (Μ) OF THE TISSUES AND CT NUMBER IS GIVEN AS FOLLOWS:  ?WHAT IS DISPLAYED IN CT IMAGES S. Jastaniah, J. Al-Ghamdi, A. Aref (KAU), A. Musa (KKU), K. Hassan (BMC) 05/31/2024  LINEAR ATTENUATION COEFFICIENT FOR SOME TISSUES OF THE BODY Linear Attenuation Coefficient (cm−1) Air 0.0003 Fat 0.162 Water 0.180 White matter 0.187 Blood 0.182 Dense bone 0.46 Values may slightly vary according to way it was calculated in different references. S. Jastaniah, J. Al-Ghamdi, A. Aref (KAU), A. Musa (KKU), K. Hassan (BMC) 05/31/2024 4   WATER IS USED BECAUSE IT IS IN ABUNDANCE IN THE BODY AND HAS A UNIFORM DENSITY.  (WATER) HAS BEEN CHOSEN AS THE REFERENCE MATERIAL FOR CT NUMBERS AND ASSIGNED A CT VALUE OF ZERO.  TISSUE MATERIALS WITH DENSITY (OR ATTENUATION) GREATER THAN WATER HAVE +VE CT NUMBERS. THOSE LESS DENSE THAN WATER WILL HAVE –VE CT NUMBERS. S. Jastaniah, J. Al-Ghamdi, A. Aref (KAU), A. Musa (KKU), K. Hassan (BMC) 05/31/2024 5   A HOUNSFIELD UNIT (H.U.) IS A DIGITAL VALUE ASSIGNED TO EACH PIXEL.  THESE UNITS LIE ON A SCALE WHERE WATER HAS A VALUE OF (0), AIR HAS A VALUE OF (–1000),AND BONE HAS A VALUE OF (+1000). S. Jastaniah, J. Al-Ghamdi, A. Aref (KAU), A. Musa (KKU), K. Hassan (BMC) 05/31/2024 -   COMPUTER RECONSTRUCTION THEN PRODUCES LINEAR ATTENUATION VALUES (Μ) RELATIVE TO THAT OF WATER AND SCALED TO A LARGE NUMBER KNOWN AS THE CT NUMBER: CT NUMBER = [ (ΜTISSUE – ΜWATER)/ ΜWATER ]. K (K ) IS A CONSTANT FACTOR.  VALUE OF K DETERMINES A CONTRAST FACTOR (SCALING FACTOR). S. Jastaniah, J. Al-Ghamdi, A. Aref (KAU), A. Musa (KKU), K. Hassan (BMC) 05/31/2024 -  CT NUMBER S. Jastaniah, J. Al-Ghamdi, A. Aref (KAU), A. Musa (KKU), K. Hassan (BMC) 05/31/2024 8   CT NUMBERS ARE ESTABLISHED ON A RELATIVE BASIS WITH THE ATTENUATION OF WATER (= 0.19) AS A REFERENCE AND THE SCALING FACTOR (K) OF THE SCANNER IS 1000.  THUS CT NUMBER FOR WATER IS ALWAYS = ZERO, CALCULATED AS FOLLOWS: CT WATER = [(ΜWATER – ΜWATER) / ΜWATER]. K = [(0.19 – 0.19) / 0.19]. 1000 = 0 S. Jastaniah, J. Al-Ghamdi, A. Aref (KAU), A. Musa (KKU), K. Hassan (BMC) 05/31/2024 9  CT numbers for common tissues CT Number (HU) Tissue 1000 – Air to – 300 900 – Lungs to –80 120 – Fat 0 Water to 30 10 Muscle To 100 50 Soft tissue 05/31/2024 - S. Jastaniah, J. Al-Ghamdi, A. Aref (KAU), A. Musa (KKU), K. Hassan (BMC)  :IMAGE DISPLAY  THE FINAL STEP IN THE CT PROCESS INVOLVES IMAGE DISPLAY, STORAGE, AND DOCUMENTATION. AFTER THE CT IMAGE HAS BEEN RECONSTRUCTED, IT EXITS THE COMPUTER IN DIGITAL FORM  THIS MUST BE CONVERTED TO A GRAY SCALE IMAGE FOR INTERPRETATION BY THE RADIOLOGIST.  THE GRAY SCALE IMAGE IS DISPLAYED ON A TELEVISION MONITOR, WHICH IS AN ESSENTIAL COMPONENT OF THE CONTROL OR VIEWING CONSOLE. S. Jastaniah, J. Al-Ghamdi, A. Aref (KAU), A. Musa (KKU), K. Hassan (BMC) 05/31/2024 11  DISPLAYING THE IMAGE  THE DISPLAY DEVICE IS USUALLY LCD (LIQUID CRYSTAL DISPLAY) , EACH PIXEL WITHIN THE IMAGE IS ASSIGNED A LEVEL OF GRAY.  THE GRAY LEVEL ASSIGNED TO EACH PIXEL CORRESPONDS TO THE CT NUMBER OR HOUNSFIELD UNITS FOR THAT PIXEL. S. Jastaniah, J. Al-Ghamdi, A. Aref (KAU), A. Musa (KKU), K. Hassan (BMC) 05/31/2024 12 S. Jastaniah, J. Al-Ghamdi, A. Aref (KAU), A. Musa (KKU), K. Hassan (BMC) 05/31/2024 13  DISPLAY MONITORS (CONT’D)  CRT (CATHODE-RAY TUBE ) MONITORS ARE HEAVIER, LARGER, HOTTER, AND LESS DURABLE THAN THE NEWER LCD MONITORS.  LCD MONITORS PRODUCE HIGHER LUMINANCE AND HIGH SPATIAL RESOLUTION. S. Jastaniah, J. Al-Ghamdi, A. Aref (KAU), A. Musa (KKU), K. Hassan (BMC) 05/31/2024 14  CT IMAGE MATRIX  THE DIGITAL CT IMAGE IS DISPLAYED ON ARRANGEMENT OF NUMERICAL VALUES CALLED MATRIX. THE MATRIX IS MADE UP A SERIES OF PICTURE ELEMENT, ARRANGED IN ROWS AND COLUMNS, CALLED PIXELS.  A VOXEL (VOLUME ELEMENT) REPRESENTS A VOLUME OF PATIENT DATA OR SLICE THICKNESS GIVES THE PIXEL AND ADDED DIMENSION. S. Jastaniah, J. Al-Ghamdi, A. Aref (KAU), A. Musa (KKU), K. Hassan (BMC) 05/31/2024 15  EACH MATRIX CELL IS A PICTURE ELEMENT (PIXEL) EACH PIXEL IS A TWO DIMENSIONAL REPRESENTATION OF A VOLUME ELEMENT (VOXEL) S. Jastaniah, J. Al-Ghamdi, A. Aref (KAU), A. Musa (KKU), K. Hassan (BMC) 05/31/2024 16  THE DIAMETER OF THE RECONSTRUCTED IMAGE IS THE FIELD OF VIEW (FOV) MATRIX SIZE DESCRIBES THE NUMBER OF PIXELS IN AN IMAGE S. Jastaniah, J. Al-Ghamdi, A. Aref (KAU), A. Musa (KKU), K. Hassan (BMC) 05/31/2024 17