[Oradlist] Hounsfield Units and x-ray beam energy

[feraso at aol.com]

Roberto:

I just came from vacations, spoke with Olga and German Aguilar in Colombia and they are very happy with the machine.  As I remember you wanted some images from my iCAT, please remind me what you ask for, and sorry for the delay,



FRANCISCO


-----Original Message-----
From: Roberto Molteni <rmolteni at afpimaging.com>
To: Oral Radiology Discussion Group <oradlist at lists.ucla.edu>
Sent: Mon, 3 Dec 2007 5:22 pm
Subject: [Oradlist] Hounsfield Units and x-ray beam energy





My posting of two months ago (Sept. 13th, re. CBCT Bone Density Measurements) about Hounsfield Units opened a can of worms!

It looks like I made a mistake. Apparently, the definition of Hounsfield Units does NOT include an (explicit) specification of the beam energy at which the H.U. are measured.

Of course, since Hounsfield Units is not a recognized Unit of Measure in the S.I. (Système International d'Unités), it does not benefit from a rigorous and formal definition, and rather relies on a consensus definition in the scientific literature. Which does not (usually) indicate a reference energy for the x-ray beam.

However, it ought to !!

All literature concurs that the x-ray beam energy (viz. the kV) influences the relative signal at the different tissue densities. In various works about comparison of H.D. from different systems and/or at different conditions, this is accounted for with an "energy correction factor" in the calculations. This implies that only at one given (monochromatic) beam energy no correction is required.

Nonetheless, my argument was not completely groundless. In "Computed Tomography-Physical Principles, Clinical Applications and Quality Control" edit by W.B. Saunders Company, E. Seeram states (p.67) "in the original CT scanner, CT numbers were calculated on the basis of 73 keV'" (not 63 keV as I had incorrectly indicated). Beam energy of 70 keV is also postulated for the linear attenuation coefficients provided in AAPM Report N. 1 (1997) "Phantoms for performance evaluation and quality assurance of CT scanners". The range 60-75 keV is consistent with the beam peak energy obtained with the typical filtration and kV of many dental CBCT systems.

Comments are appreciated.

 

Roberto Molteni
AFP Imaging / Dent-X / NewTomDental / QR

 




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