shades of grey

1. -DR. SANDRA JOHNS

2.  Matrix
 Linear attenuation coefficient
 CT number and Unit
 Window Level and Window Width
 Different grey scales
 CT post contrast enhancement

3.  The image is represented as matrix of numbers
 The 2D array of numbers arranged in rows or
columns
 3D-voxel
 2D-pixel

4.  Fraction of a beam of x-rays that is absorbed or
scattered per unit thickness of absorber
-CT images are a display of amount of attenuation
when x ray beam penetrate body

5.  The density measurement of this attenuated
beam is assigned a number.
 Each pixel has a number which represents the
xray attenuation in corresponding voxel of
object.

6.  This CT Number is
compared with
attenuation value of
water and displayed on
a scale of units called
Hounsfield unit.
 This scale assigns water
as an attenuation
value(HU) as zero.
 CT number-2000
 Where +1000 (white)to -
1000(black) at either end
of spectrum

7.  Window level –central
HU of all the numbers
within the window
width
 Window width-covers
HU of all the tissues
of interest as shades of
gray(CT number
outside as black
/white)

9. HYPODENSE HYPERDENSE
 AIR
 FAT
 FLUID
 GLIOSIS
 VASOGENIC AND
CYTOTOXIC
EDEMA
 BONE
 ACUTE BLEED
 METALLIC IMPLANTS
 CALCULI-RENAL
ISODENSE
 SUBACUTE
HAEMATOMA

11. HYPERENHANCING
 VASCULAR
PATHOLOGIES
 TUMOURS
HYPOENHANCING
 ABSCESS
 CYST
 HYPOENHANCING
TUMOURS-
PANCREATIC
TUMOURS EXCEPT
NEUROENDOCRINE
TUMOURS
 NECROTIC MASSES