week 11 DICOM, PACS etc.

DICOM (digital imaging and communications in medicine)

an international standard (ISO) for handling, storing, printing, and transmitting information in medical imaging

includes a file format definition and a network communication protocol

communication protocol is an application protocol that uses TCP/IP to communicate between systems

what does a single DICOM file contain?

a header (which stores information about the patients name, type of scan, image dimension, etc.)

image data (in compressed or uncompressed form)

PACS (picture archiving and communication system)

an integrated computer system for the storage, transfer and display of radiological images

consists of 

• a digital archive to store medical images

• display workstations to permit physicians to view the images

• a computer network to transfer images and related information between the imaging devices and the archive and between the archive and the display workstatio

point spread function (PSF)

most basic measure of the resolution properties of an imaging system

response of the imaging system to a point input 

a two dimensional function, described in the x and y dimension

line spread function (LSF)

an imaging system is simulated with a signal in the form of a line is used to evaluate LSF

a profile measured perpendicular to the line gives LSF(x)

most common method for measuring blur and system resolution (FWHM)

magnification

results in increased penumbral blurring (geometric unsharpness)

magnification = source to image distance / source to object distance

depends on focal spot size, SOD, SID, OID

focal spot size effect on magnification / spatial resolution

the smaller the focal spot, the sharper the image - better spatial resolution

but with smaller focus, only smaller mA can be used - long exposure time

fourier transform (FT)

FT breaks any arbitrary signal down into the sum of a set of sine waves of different phase, frequency, and amplitude

transforms spatial domain signal to frequency domain

used for filtering procedure in filtered back projection for CT reconstruction

modulation transfer function (MTF)

provides the most complete characterisation of the resolution of an imaging system

how well the contrast is transferred from object to image

MTF = image contrast/object contrast

limiting resolution

characterises the approximate resolution limit (a single number) of an imaging system

considered to be the frequency at which the MTF crosses the 10% level

what is the nyquist frequency?

highest frequency that can be accurately measured on the imaging system

for an imaging system, FN =1/ 2Δ, where Δ (mm) is the centre-to-centre spacing between detector elements

what happens if the input frequency is higher than the nyquist frequency (F_N)?

the true frequency will not be recorded - it would be aliased, and the frequency recorded will be lower than the incident 

the measured frequency will be lower than FN by the same amount which the input exceeds FN

what is contrast resolution characterised by?

Signal to Noise Ratio (SNR) - number of photons (mA)

what types of noise is contrast resolution affected by?

quantum - fluctuation in x-ray, photons, electrons, etc

electronic - exists in all electronic circuits

anatomical - unwanted anatomy

signal to noise ratio (SNR)

as signal increases, SNR increases (image quality increases)

SNR = N (square root)

i.e. if the signal is doubled, dose is doubled but SNR increases by square root of 2

to double SNR, dose must be increased by a factor of 4

what is a good measure of the dose efficiency of an x-ray detector system?

detective quantum efficiency (DQE)

what does the DQE describe?

how effectively an x-ray imaging system can produce an image with high SNR