INTRODUCTION TO IMAGING PHYSICS CAPABILITIES AND LIMITATIONS

SPECTRUM OF E-M RADIATION

GENERATION OF X-Rays VACUUM TUBE Electric current is passed through a filament, leading to e - emission, then striking target (W or Mb), leading to X-ray emission.

SCATTERING

How reduce X-ray SCATTERING ? ASK YOUR PATIENTS TO LOSE WEIGHT?

X-ray Collimation

X-ray GRID Tradeoff Grids require  mAs compared with XR studies done w/o grids

How Improve Spatial Resolution & Decrease Image Distortion?

Center the Area of Interest!

FIRST APHORISM DON’T MAKE GOOD CALLS FROM “BAD” FILMS !!

DON’T MAKE GOOD CALLS FROM “BAD” FILMS !!! * “ Bad” can mean Suboptimal Quality OR the study as ordered was NOT dedicated for evaluation of Region or Organ of Interest.

SECOND APHORISM YOU CANNOT CALL WHAT YOU DON’T SEE! * HOWEVER, IF YOU SUSPECT SOMETHING, GET ANOTHER VIEW!! *

DIGITAL/COMPUTED RADIOGRAPHY IMAGES CAN BE MANIPULATED POST-ACQUISITION TO OPTIMIZE VIEWING OF ONE PART OF H&D Curve.

2 nd Generation CT scanner

%Transmission Special Case For monochromatic Photon energy – log %T α 1/linear attenuation

What data generates an image as a slice? The %Transmission of Photon energy received by detectors is recorded at multiple projections around the subject & the data is then reconstructed to create a cross-sectional image

X-ray Attenuation Revisited * %Transmission of photon energy received by detectors is recorded at multiple projections around the subject & the data is reconstructed to create a cross-sectional image

ATTENUATION VALUE – CT * Hounsfield Units (H.U.) * of sample S = ( μ S - μ H2O ) x 1000 μ H2O

CT BEST FOR Calcification e.g. a Bony Sequestrum & Involucrum of Osteomyelitis

PARTIAL VOLUME EFFECT

EFFECT OF THICK SLICES

BEAM-HARDENING

Metallic streaking

… AND IMAGES DERIVED FROM THOSE w/ ARTIFACTS

3 RD GENERATION CT

THIN SLICES  ISOTROPIC VOXELS

Hypervascular Met only seen on Hepatic Arterial phase

MRI 3

The NMR Phenomenon

IMAGE CONTRAST POSSIBILITIES Processing of emitted RF signal yields Spatial Information as well as various forms of Image Contrast

ACKNOWLEDGEMENTS ILLUSTRATIONS COURTESY OF: MRI in Practice , 3 rd ed. Westbrook… Clinical MRI Atlas , 2 nd ed. Runge… Radiologic Physics , 4 th ed. Christenson… Fundamentals of Radiology , LF Squire