# How to get Hounsfield units from raw pixel data for CT?
**URL:** https://discourse.vtk.org/t/how-to-get-hounsfield-units-from-raw-pixel-data-for-ct/14651
**Category:** Support
**Created:** [October 3, 2024, 1:54am UTC](https://discourse.vtk.org/t/how-to-get-hounsfield-units-from-raw-pixel-data-for-ct/14651 "2024-10-03T01:54:22Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@Grigoriy\_Postolskiy](https://discourse.vtk.org/u/Grigoriy_Postolskiy)
#### Post date: [October 3, 2024, 1:54am UTC](https://discourse.vtk.org/t/how-to-get-hounsfield-units-from-raw-pixel-data-for-ct/14651/1 "2024-10-03T01:54:22Z")
Hello! When I read the value of individual pixels of CT or MRI, I get values in the range from 0 to 65535. How can I get values in Hounsfield units, that is, in the range from -1024 to ~2000?
For example:
- contrasted vessels: 300-500 HU
- the lungs: about -750 HU
- etc.
---
### Author:  [@constifer](https://discourse.vtk.org/u/constifer)
#### Post date: [October 4, 2024, 7:48pm UTC](https://discourse.vtk.org/t/how-to-get-hounsfield-units-from-raw-pixel-data-for-ct/14651/2 "2024-10-04T19:48:54Z")
Get the slope and intercept from the header of the file then you could write a function or create a look up table to generate values based on: HU = Gray\_Value \* slope + intercept.
---
### Author:  [@Grigoriy\_Postolskiy](https://discourse.vtk.org/u/Grigoriy_Postolskiy)
#### Post date: [October 9, 2024, 3:38am UTC](https://discourse.vtk.org/t/how-to-get-hounsfield-units-from-raw-pixel-data-for-ct/14651/3 "2024-10-09T03:38:35Z")
I found the solution. The conversion formula is:
```auto
HU = (raw_pixel - PixelRepresentation * 2^BitsAllocated / 2)
>> (BitsAllocated - BitsStored) * RescaleSlope + RescaleIntercept
```
Firstly, is need to make bit shifting to right.
- (0028,0100) BitsAllocated - the count of bits for one pixel storing
- (0028,0101) BitsStored - the count of bits that really used from allocated
Example:
BitsAllocated - BitsStored = 16 - 12 = 4, so we need to shift raw pixel data to 4 to rigth (this is equal to dividing by 16).
Also the important tag is:
(0028,0103) PixelRepresentation
if is equal to 0: unsigned pixel data
if is equal to 1: signed data
If PixelRepresentation equals to 1 (signed pixel data), then we need subtract the half from max value that can be stored in the pixel. For example, if BitsAllocated = 16, then 65536 - is the maximum number of values that can be stored in 16 bits. Then the half would be 32768.
Example 1:
- min raw pixels: 0
- max raw pixels: 49936
- min HU: -1024
- max HU: 2097
- (0028,0100) BitsAllocated: 16
- (0028,0103) PixelRepresentation: 0
- (0028,0101) BitsStored: 12
- (0028,1053) RescaleSlope: 1
- (0028,1052) RescaleIntercept: -1024
The answer is:
- min HU = (0 - 0 \* 2^16 / 2) \>\> (16 - 12) \* 1 - 1024 = -1024
- max HU = (49936 - 0 \* 2^16 / 2) \>\> (16 - 12) \* 1 - 1024 = 2097
Example 2:
- min raw pixels: 30768
- max raw pixels: 35679
- min HU: -3024
- max HU: 1887
- (0028,0100) BitsAllocated: 16
- (0028,0103) PixelRepresentation: 1
- (0028,0101) BitsStored: 16
- (0028,1053) RescaleSlope: 1
- (0028,1052) RescaleIntercept: -1024
The answer is:
- min HU = (30768 - 1 \* 2^16 / 2) \>\> (16 - 16) \* 1 - 1024 = -3024
- max HU = (35679 - 1 \* 2^16 / 2) \>\> (16 - 16) \* 1 - 1024 = 1887
Example 3:
- min raw pixels: 31744
- max raw pixels: 34311
- min HU: -1024
- max HU: 1543
- (0028,0100) BitsAllocated: 16
- (0028,0103) PixelRepresentation: 1
- (0028,0101) BitsStored: 16
- (0028,1053) RescaleSlope: 1
- (0028,1052) RescaleIntercept: 0
The answer is:
- min HU = (31744 - 1 \* 2^16 / 2 \>\> (16 - 16) \* 1 + 0 = -1024
- max HU = (34311 - 1 \* 2^16 / 2) \>\> (16 - 16) \* 1 + 0 = 1543