Physics of Fluoroscopy

School of MedicineDIVISION OF MEDICAL PHYSICSCardiac X-ray ImagingAndrew DaviesLecturer in Medical ImagingDivision of Medical Physics© Copyright, Andrew Davies/The University of Leeds, 2012Do not reproduce this material without permission

Contents1. X-ray imaging fundamentals2. X-rays cardiac imaging and the cardiaccatheterisation laboratory3. Characteristics of cardiac X-ray images4. Example image sequences5. X-ray dose6. Summary7. Bibliography

Why image with X-rays?X-ray imaging can be used to detect soft tissue abnormalities,image bone, and with the use of contrast agents image thecadiovascular, GI and other internal structures.It can also be used to visualise devices within the body asthey are manipulated during interventions.In cardiac imaging X-ray imaging plays a key role in both thediagnosis and treatment of coronary heart disease, andother cardiac conditions.

X-RAY PRODUCTION

Producing X-raysX-rays are produced by colliding electrons into a metal targetin an evacuated X-ray tube.Electrons are produced by passing a current through afilament, heating the filament, resulting in electron emission.The electrons are accelerated towards a metal target anodeby the application of a potential difference typically 40 – 120kV for cardiac imaging.Anode+ -e -X-ray photonsCathodeFilament coil& focussing cup

Producing X-raysInteraction between the electronsand the target produce X-rays asBremsstrahlung andcharacteristic radiation.Tungsten anodeX-ray tube output with3.5 mm Al inherent filtration50 kV p12 x 104 Energy (keV)A polyenergetic radiation beam isproduced.An X-ray generator isresponsible for producing theelectrical supply to the X-raytube.X-ray production is veryinefficient (0.9 x 10 -9 ZV).N photons per mm 2 per mAs10864270 kV p90 kV p00 10 20 30 40 50 60 70 80 90 100

X-RAY INTERACTIONS

X-ray interactions with matter1. Photoelectric AbsorptionAn X-ray photon interacts with aninner electron of an atom,transferring all of its energy to theelectron.The electron is ejected from theatom and the electron hole is normallyfilled by a series of electron cascadesfrom outer shell electrons, resulting inauger electron and characteristicphoton emissions. The X-ray photonis effectively removed from thebeam.Chance of photoelectric absorptionproportional to (Z/E p ) 3 .Ejected electronIncident X-ray photonCharacteristic X-rayand/orAuger electron

X-ray interactions with matter2. Compton (Incoherent) ScatterAn X-ray photon interacts with aloosely bound or free electron,transferring part of its energy to theelectron.The X-ray photon is deflected, butis not removed from the beam.Scattered X-ray photons can:• further interact with the patient, or• escape the patient and interact withother objects such as the X-ray imagereceptor (degrading image quality) ormembers of staff.Incident X-ray photonEjectedelectron

IMAGE FORMATION

Image formationModern X-rayimage receptorsare highly efficientsolid state (“flatpanel”) devices.X-raysPolished Al coatingintensityCsI scintillatorconverts x-rayphotons to lightdistancea-Si TFT photodiode array

Projection imagesX-ray images are radiographic shadowgrams- three dimensional objects areprojected onto a two dimensional image.

X-RAY CARDIAC IMAGING ANDTHE CATH LAB

The cath lab

The X-ray cath Lab

Coronary X-ray image sequencesFeature Typical OptionalTemporal resolutionSpatial resolutionImaging modeField of viewC-arm motion15 x 5 ms pulses persecond1024 x 1024 pixels20 x 20 cm receptor area(200 µm pixel pitch;100 µm effective pixelsize in object plane)30 fps also commonclinically, but both higherand lower frame ratespossibleSpecialist imagereceptors can havesmaller pixel pitchFluoroscopy & acquisition User selectable25, 20 & 15 cm±45° C-C, ±120° Lat18° - 25°/s motorisedmotion

EXAMPLE IMAGES

Percutaneous Coronary InterventionBlood has a very similar linear attenuationco-efficient to soft tissue, and therefore isvery difficult to directly discriminate using X-ray imaging.Blood is therefore temporarily replaced byan iodinated contrast medium with a muchgreater attenuation than blood for imaging.A catheter is inserted into radial or femoralartery and advanced over the aortic arch tothe aortic root. It is then selectively engagedwith the ostium of the right or left coronaryartery, allowing the injection of contrastmedium or placement of interventionaldevices into the artery.

StentingA stent is a metal mesh support structure placed in the arteryto reduce the rate of restenosis.The stent is positioned underfluoroscopic guidance…… and inflated to expand thestent. The balloon is thenwithdrawn.

Post intervention angioPost intervention angiogramsare then acquired in order toconfirm proper deployment,and restoration of vessellumen.More than one stent can beplaced in the same artery, orfurther arteries treated asnecessary.

Fluoroscopy (EP)

3D imaging in the cath lab1221

3D model

Radiation DoseDose rates on the image receptor dictate the level of noisewithin the image (noise ∝ √dose).X-rays have two main detrimental effects to humans:• Deterministic effects, mainly damage to the skin• Threshold (2 Gy) before any effect seen• Severity increases with dose• Stochastic effects, fatal cancer and birth defects• Severity not affected by dose• Likelihood of occurrence is related to dose

Radiographic imaging of the heartFeatureHigh temporal and spatialresolution possibleProjection geometryAnatomy: accurateassessment of morphologyFunction: assessment ofblood flow / perfusionNotesLimited in humans to acceptable patient radiationdose levelsProjection information does not resolve 3Dinformation directly, but requires less X-ray dosethan cardiac CT.Aterial, veneous and ventrical lumen all possible.Morphological information of vessel lumen stenoticseverity does not necessarily correlate withfunctional severity.Blood flow is normally measured via anotherdevice (e.g. intra-arterial doppler). Has beendemonstrated directly from imaging, but is notroutine (or straightforward). Assessment ofperfusion currently rudimentary, but remains anarea of active research.

Perfusion assessmentPre interventionPost intervention

BibliographyRadiological Physics• Bushberg JT, Siebert JA, Leidholdt E, Boone JA. 2012. The EssentialPhysics of Medical Imaging. 3 rd Edition. Lippincott Williams & Wilkins.• Huda W. 2009. Review of Radiological Physics. 3 rd Edition. LippincottWilliams and Wilkins.Clinical Applications• Lanzer P (Ed.) 2012. Catheter Based Cardiovascular Interventions: aKnowledge based approach. 2012 Edition. Springer.Questions• a.g.davies@leeds.ac.uk