TIFF vs DICOM?

[Nic]

Hi,

I suppose this is really a general Image J question but as most people here use DICOM I thought I would ask it here.

I was under the impression that the information stored in TIFFs was unsigned integers and that DICOM files image files contain the original Hounsfield Unit values which are signed integers.

But I have cropped a stack of DICOM images and not being able to write our a stack of DICOMs I outputted a stack of TIFFs. When I re-opened the TIFFs I still get the exact same grey-scale values even the negative values!

How is this possible? (by the way I'm not complaining, this is very good news)

Thanks

Nic

[Abel, Richard L]

Dear Nic,

DICOM and TIF files are essentially the same. Just a sting of grey values. Usually the only difference is the header file.

The grey values in DICOM and TIF are usually arbitrary. Unless the grey values have been calibrated and converted to Hounsfield units. A step which is only usually used in Medical-CT scans.

Best Wishes
Richie

_____________________________
Dr. Richard L. Abel
Lecturer in Musculoskeletal Science
Imperial College, London.

---

From: bonej-users-a...@googlegroups.com [bonej-users-a...@googlegroups.com] on behalf of Nic [dingt...@googlemail.com]
Sent: 26 July 2012 19:28
To: bonej-users-a...@googlegroups.com
Subject: [BoneJ: 236] TIFF vs DICOM?

[Michael Doube]

> I was under the impression that the information stored in TIFFs was
> unsigned integers

The pixels can be of many types in TIFF. Signed or unsigned integer,
floating-point, big- or little-endian.

> and that DICOM files image files contain the original
> Hounsfield Unit values which are signed integers.

They might in some cases, but it's not a requirement of the DICOM
standard. DICOM is also used by e.g. MRI scanners for which Hounsfield
units are meaningless.

> But I have cropped a stack of DICOM images and not being able to write
> our a stack of DICOMs I outputted a stack of TIFFs. When I re-opened the
> TIFFs I still get the exact same grey-scale values even the negative values!

Often what there is is the raw pixel data and a calibration function. In
the status bar of ImageJ you will see two numbers, like -977.00 (23)
when hovering the cursor over pixels representing a CT scan of air. What
this means is that the raw pixel value 23 is being shifted by -1000 to
give an HU value of -977, by the calibration function y = x - 1000.

ImageJ handles opening DICOM fine, but will save as TIFF with the DICOM
tags, including calibration, entered in the TIFF header. You can see the
header with [i].

Michael