RADIOGRAPHY

Ho Kyung Kim, Ph.D.hokyung@pusan.ac.kr

School of Mechanical EngineeringPusan National University

Introduction to Medical Engineering

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• Discovered x rays by Wilhelm Konrad Röntgen in 1895– While experimenting with a Crookes tube discovered that a plate of Barium Platinum Cyanide did

glow when he activated the tube. Even when he covered the tube with black material it kept glowing. In the next experiments he used photographic material and made his first x-ray picture.

• Established the nature of x rays as short-wave EM radiation by Max von Laue in 1912

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X rays

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• X rays are electromagnetic waves, consisting of photons• Relationship; energy E, frequency f, and wavelength ;

– h = Plank's constant (= 6.626 10-34 Js = 4.135 10-15 eVs)– c = speed of light (= 3 106 m/s)– ~ Å (Angstrøms, 10-10 m)– E ~ keV

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X-ray generation

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• Typically generated x rays in an x-ray tube, consisting of a vacuum tube with a cathodeand anode

– Released electrons (proportional to the cathode current J) at the cathode by thermal excitation– Accelerated electrons toward the anode by a voltage U b/w the cathode and the anode– Hit the anode and released their energy by producing

• bremsstrahlung (continuous spectrum)• characteristic radiation (discrete spectrum; specific peaks arise at specific orbital shell

energies)

• Important x-ray tube parameters– the amount of emitted photons (= the amount of electrons hitting the anode) = the cathode

current (mA) the time the current is on (s) = mAs• typically 1 – 100 mAs

– the energy of emitted photons (in keV) by the voltage b/w cathode and anode (kV) (= the energy of the electrons hitting the anode)

• typically 50 – 125 kV (mammography 22 – 34 kV)– the total incident energy (J = kV mA s)

• x rays 1% & heat 99%

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Bremsstrahlung radiation

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Characteristic radiation

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Interaction of photons with matter

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• Photons with energy 13.6 eV are non-ionizing, and the consequence of interaction is “excitation”.

• Rayleigh scattering (or coherent scattering)– Non-ionizing process occurring at lower energies (< 30 keV)– Lower the energy higher scattering angle

• Photoelectric absorption– Absorbed an incident photon by an atom while its energy excites an electron– Escaped the electron from its nucleus in the same direction with the incident photon

• Compton scattering (or incoherent scattering)– Transferred only part of the incident photon energy to an electron– Emitted a photon of remaining lower energy in the deviated direction– Escaped the electron in another direction

• Pair production– Transformed the incident photon ( 1.02 MeV) into electron-positron pair– Annihilated the positron with another electron each other while creating two 511-keV photons

flying-off in opposite directions

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Interaction of an x-ray beam with tissue

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• Considering photons with a single energy impinging to the homogeneous material;

– = linear attenuation coefficient in cm–1 , function of (E, material)

• For nonhomogenous materials;

• More generally considering the polyenergetic x-ray beam;

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• PE at low energies• CS at intermediate energies• PP at very high energies• More rapid decrease of PE than CS as E • with Z (more rapid increase of PE

than CS)

• Mass attenuation coefficient in cm2/g

• Why do we call linear coefficients?

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Screen-film detector

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• Very inefficient photographic film for capturing x rays– Only 2% of the incoming x rays contributes to the output image (quantum absorption efficiency)– Would yield prohibitively large patient dose– Typically, placed the film b/w two intensifying screens

• Screen– Containing phosphors (Gd2O2S:Tb) with a high quantum absorption efficiency– Absorbing most of the x-ray photons– 25% of QAE or QE of each screen instead of 2% for film– Converting x rays into visible light (which is scattered in all directions, resulting in image blur)– Fluorescence

• prompt emission & stop of light and used in intensifying screens• CaWO4, Gd2O2S:Tb, CsI:Tl

– Phosphorescence (or afterglow)• continuation of light emission (> 10-8 s)• undesirable because it causes “ghost” images and image lag (and fogging in film)

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• Film– Containing an emulsion with silver halide crystals (e.g., AgBr)– Absorbed optical photons by the silver halide grains, and then

metalized (dark)– Precipitated metallic silver when developed– Negative image

– Graininess• The image derived from the silver crystals is not continuous but

grainy• The larger the grains, the faster the film becomes dark (amount

of photons needed to change a grain into metallic silver upon development is independent of the grain size)

– Speed• Inversely proportional to the amount of light needed to produce

a given amount of metallic silver on development• Mainly determined by the silver halide grain size• The larger gain size the higher the speed• How many x-ray photons are needed to produce a certain

density on the film• Speed in the screen-film system: Reflector improves the speed

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– Contrast• Plot of the optical density D vs. the logarithm of the

exposure E (called the sensitometric curve)

• A larger slope implies a higher contrast at the cost of a smaller useful exposure range

• gamma: the maximal slope

– Resolution• Depending on its grain size and the light scattering

properties

film developingwhen intensity light

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A. A light photon removes the outermost electron from a bromide anion. The bromine atom (now uncharged) diffuses out the crystal. The liberated electron wanders through the crystal and is trapped at the sensitivity speck.

B. The speck is now negatively charged.C. It draws an interstitial silver cation to itself.D. The electron on the sensitivity speck

neutralizes the charge of the silver ion, and the resulting silver atom is deposited there.

E. Another light photon causes the process to repeat. The deposition of 10 or so silver atoms at the sensitivity speck transforms it into a latent image center. A crystal with a latent image center will be transformed into a fleck of pure silver during the development process.

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no intensifying screen128 mAs, >12 lp/mm,

fine screen10 mAs, >7 lp/mm,

fast screen1.33 mAs, <5 lp/mm,

Image intensifier

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• Working principle– Conversion of x rays into visible light by an input phosphor (or fluorescent) screen– Emission of electrons from a photocathode hit by light– Accelerated the ejected electrons by a potential difference b/w the cathode and the output– Focused electron beam to the output phosphor screen by electrostatic or magnetic focusing– Captured visible light from the phosphor screen by a camera

• Capable of producing dynamic image sequences in real time at video rate (a process known as fluoroscopy)

• Image degradation– Less spatial resolution rather than that of a film-screen system (because of the limited camera

resolution)– Increased noise due to the additional conversions (light electrons light)– Geometric distortion, called pin-cushion distortion, toward the borders of the image

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Storage phosphors

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• Also called photostimulable phosphors• Photo-stimulated luminescence

– An extreme case of phosphorescence– Released the temporarily stored energy in a form of light by stimulation (laser)

• Computed radiography– Use of the storage phosphor– Trapped the excited electrons by electron traps (impurities in the scintillator)– (it takes 8 h to decrease the stored energy by ~25%)– Extraction of stored energy or latent image by pixelwise scanning with a laser beam– Released visible light by the de-excitation of electrons– Captured light by an optic array and transmitted to a photomultiplier– Converted analog electrical signal into a digital bit stream by an A/D converter– Erased any residual image by a strong light source

• Advantages– Wider useful exposure range than film-screen systems– Linear response (tolerant to over- & under-exposure, and a reduced retakes)– Dose reduction due to the available contrast at low exposure– Allows post-image processing and digital storage– Allows the use in PACS (picture archiving and communication system)

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Flat-panel detectors

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• CCD (charge-coupled device)– Based on Si-crystal technology, thus limited to small areas

• Hydrogenated amorphous silicon– Can be deposited on large-area substrates

• CsI-based flat-panel detector– CsI:Tl

• Excellent quantum efficiency and good resolution (need-like structure)– a-Si:H photodide/TFT array for reading out optical photons from CsI:Tl

• Se-based flat-panel detector– Based on amorphous selenium photoconductor

• Applied homogeneous electric charge to the surfaces before exposure• Generated e-h pairs and passed to the surface of a-Se, where they neutralize a part of the

applied charge• Locally reduced the surface charge, thus producing a latent image

– a-Si:H storage capacitor/TFT array for reading out the latent image

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Image Courtesy of GE HealthCare

Image Courtesy of Anrad

Readout pixel array

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Chare-sensitive amplifiers

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Digital signals

Analog signals

Bias

Image Courtesy ofSamsung Electronics Co. & Vatech, Co., Ltd.

• Unlike the direct-conversion configuration, the optical coupling between the scintillators and the readout pixel arrays in the indirect-conversion scheme is important because, for example, the mismatch of refractive indices between the two components results in a significant loss of optical photons, reducing the x-ray sensitivity.

Signal spreading

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Does signal spreading always affect badly?

• How can you interpolate missing pixels without signal spreading?

• No signal spreading, thus an extended MTF is absolutely detrimental to the detectability of faint details (due to noise aliasing).

– Hologic dropped their chest DR business in 2007, instead is focusing on mammography DR.

• Should the MTF be as large as possible below the Nyquist limit, and then drop rapidly, which is the ideal.

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Dual-energy imaging

• Tissue-selective imaging• Typical configurations

– Fast kVp-switching method with DR detectors• two radiographic images in a short time (e.g. 200 ms) at different kVp (e.g. 110–150 kV &

60–80 kV)– Sandwich detector: front scintillator/photo-detector/back scintillator

• front: measuring low-energy photons / back: measuring high-energy photons– Multi-energy photon-counting detector

• single shot, insensitive to patient motion (motion artifact)

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• Dual-energy imaging relies on (E)

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The attenuation coeff. of an arbitrary substance Scan be written as (a linear combination of the atten. coeff. of two selected materials):

Then, the intensity at pixel (x, y) is given by

Assuming that

We have then,

To determine, A1 and A2, we need two equations such that

When using a sandwich detector

Lx,y = the projection line arriving at pixel (x, y)

represent the equivalent thickness of the basis materials along ray Lx,y => unknowns

Solve two nonlinear equations (approximation to linear systems or numerical approaches)

K-edge imaging requires a third unknown A3.

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Image quality

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• Resolution– Focal spot size of x-ray source (larger angle of the anode tip)– Scattered x rays induced by the patient (use of collimator grid)– Light scattering in the phosphor– Intrinsic resolution of detectors (film grain or pixel size)– Sampling step, laser spot size … (CR)– Screen-film system = 5 – 15 lp/mm; CR system = 2.5 – 5 lp/mm at 10% contrast

• Contrast– (E, x) and thicknesses of the different tissue layers– X-ray energy (spectrum)– Quantum absorption efficiency* of a detector– In screen-film systems, the contrast is determined by the contrast of the photographic film

• The higher the contrast, the lower the useful exposure range– In DR, contrast can be enhanced after image processing (e.g., gray level transformations), but

CNR is unchanged with increasing noise

* Fraction of the total radiation hitting the detector that is actually absorbed by it

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• Noise– Quantum noise due to the statistical nature of x rays

• Poisson process (variance = mean) noise amplitude (standard deviation) ~ (signal)– SNR ~ (signal)

• the faster the detector or the higher the speed the fewer the photons needed & the lower SNR

• hence, the dose cannot be decreased unpunished• additional conversion process in the imaging process will add noise & further decrease in

the SNR

– Detective quantum efficiency

•

• Artifacts– Scratches, dead pixels, unread scan lines, inhomogeneous x-ray intensity (heel effect), afterglow– Geometric distortions in an image intensifier

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Equipment

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• Radiographic imaging chain– X-ray source– Aluminum filter (often complemented by a copper filter)

• Remove low-energy photons (beam hardening) (why to remove?)– Collimator to limit the patient area to be irradiated– (Collimating) Scatter grid to stop photons with large incidence angle

• Not always used in paediatrics (why?)– Detector

General purpose radiographic room

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• Both an image intensifier and a storage phosphor are available.

• Siemens system with large-area amorphous silicon detector coupled to a CsI scintillator plate (which replaces the cassette and image intensifier)

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3D rotational angiography system

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• C-arm with x-ray tube and image intensifier at both ends

• Active matrix flat-panel detector

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3D image of the blood vessels

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Clinical use

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• Static (or still or radiographic) images– Skeletal x rays– Chest images (radiographs of the thoracic cavity and heart)– Mammography (image of the breasts)– Dental x rays (images of the teeth and jaw)

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Double mandibular fracture with strong displacement to the left.

Solitary humeral bone cyst known as "fallen leaf sign".

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Radiographic chest image showing multiple lung metastases.

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Dense opacity with spicular border in the cranial part of the right; histological proven invasive ductal carcinoma.

Cluster of irregular microcalcifications suggesting a low differentiated carcinoma.

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• Dynamic (or fluoroscopic) images– Interventional fluoroscopy

• responsible for the majority of fluoroscopic sequences• typically used to guide and quickly verify surgical actions, particularly in bone surgery, such

as for osteosynthesis (traumatology, orthopedics)– Angiography

• imaging of blood vessels through the injection of an iodine-containing fluid into the arteries or veins

• usually subtraction images are made by subtracting postcontrast and precontrast images– motion blurring & subtraction artifacts

• used for diagnosis of conditions such as heart ischemiae caused by plaque buildup• recently to guide minimally invasive interventions of the blood vessels, such as for vascular

repermeabilization, by radiologists, cardiologists, and vascular surgeons• barium fluoroscopy of the gastrointestinal tract after the patient swallows barium contrast

solution and/or where the contrast is instilled via the rectum• Urography (image of the kidneys and bladder) using an iodine-containing contrast fluid

• Contrast agent: a substance with a high attenuation coefficient– Intravascular (blood vessels, heart cavities) imaging– Intracavitary (kidney, bladder, etc.) imaging

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Postoperative fluoroscopic control of bone fixation with plate and screws after a complete fracture of the humerus.

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Cerebral angiogram obtained by injecting an iodine containing fluid into the arteries. The contrast dye subsequently fills the cerebral arteries, capillaries and veins.

Cerebral angiogram showing an aneurysm or saccular dilation of a cerebral artery.

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Post-injection image

Subtracted image

Motion artifacts in subtracted image

Pre-injection image

Y. Bentoutou et al., Pattern Recognition (2002)

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Double contrast (barium + gas insufflation) enema with multiple diverticula in the sigmoid colon (arrows).

Polypoid mass proliferating intraluminally (arrowhead on the spotview).

Biologic effects and safety

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• Ionization chemical changes biologic damage– cell can be destroyed– cell can lose the ability to divide– cell may divide in an uncontrolled way– cell can repair itself

• Deterministic effects– injuries to a large population of cells where repair mechanisms fail and the complete tissue is

damaged– characterized by a “threshold dose” and an increase in the severity of the tissue reaction with

increasing dose

• Stochastic effects– malignant disease and heritable effects, for which the probability but not the severity is

proportional to the dose, without any threshold• probability always exists that modifications in single cells could be lead to malignancy

(cancer) or genetic changes by deposition of energy by ionizing radiation• the frequency of cell damage and the occurrence of cancer increases, but not the severity of

the cancer with increasing dose

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• Radiation dose

– Absorbed dose, D• gray (Gy) = 1 J/kg• independent of the type of irradiation• DT = the average absorbed dose in organ or tissue T

– e.g. 5 Gy in a single exposure at the level of the eye lens can cause visual impairment due to cataract (deterministic effect)

– Equivalent dose, HT

• biologic damage varies not only with the absorbed dose but also depends heavily on the type of radiation

• DT,R = the average absorbed dose from radiation of type R in tissue or organ T; wR = the radiation weighted (or quality) factor (1 for x rays, electrons, muons; 2 for protons, charged ions; 20 for heavier particles (alpha); energy-dependent for neutrons)

• sievert (Sv) = Gy radiation quality factor• nominal risk coefficient: 1.14%/Sv for lung cancer occurred on average in 114 cases per

10000 persons per Sv

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– Radiation detriment: a concept used to quantify the harmful effects of radiation exposure in different parts of the body

– Effective dose, E• biologic damage also depends on the irradiated organ• to assess the overall radiation detriment from stochastic effects• tissue weighted sum of equivalent doses in all irradiated tissues or organs of the body

• HT = the equivalent dose in tissue or organ T; wT = the tissue weighting factor

• Sv = Gy radiation quality factor tissue weighting factor– wT = 0.12 for red bone marrow, colon, lung, stomach, breast– wT = 0.08 for gonads– wT = 0.04 for bladder, oesophagus, liver, thyroid– wT = 0.01 for bone surface, brain salivary glands, skin– wT = 0.12 for adrenals, extrathoracic region, gall bladder, heart, kidneys, lymphatic

nodes, muscle, oral mucosa, pancreas, small intestine, spleen, thymus, prostate (male)/uterus/cervix (female)

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• Effective dose– dental x-ray: 0.005 – 0.02 mSv– chest: 0.01 – 0.05 mSv– skull: 0.1 – 0.2 mSv– pelvis: 0.7 – 1.4 mSv– lumber spine: 0.5 – 1.5 mSv– mammography: 1.0 – 2.0 mSv/image (usually four images for screening)– fluoroscopy ~5 mSv: 10x for diagnosis, 100x for interventional use

• intravenous urography: 3 mSv• barium enema: 8 mSv• endoscopic retrograde cholangiopancreatography (4 mSv)• interventional procedures in angiography room or catheterization lab: reaching the

thresholds for deterministic effects for skin dose– cerebral angiography: 5 mSv– transjugular intrahepatic portosystemic shunt procedures (TIPS): 70 mSv

– note that the dose equivalent due to natural sources: 2 – 3 mSv/yr

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• ICRP (International Commission on Radiological Protection)– the relative radiation detriment adjusted nominal risk coefficient for cancer

• 5.5%/Sv (4.1%/Sv for adults (18-64 yrs))• 0.2%/Sv for heritable effects up to the second generation (0.1%/Sv for adults)

– ALARA (as low as reasonably achievable)• no dose limits for patients but every exposure should be justified

– Limiting dose• Public = 1 mSv/yr• Workers = 20 mSv/yr (when averaged over five years)

Future expectations

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• Replaced x-ray examinations by US, CT, & MRI– arthrography ( joints)– myelography (spinal cord)– cholangiography (bile ducts)– cholecystography (gall bladder)– pyelography (urinary tract)

• Flat-panel detector is becoming available for 3D imaging– 2D projective imaging will further be augmented by 3D volumetric imaging– Expected the improved DQE of the detectors for reduced radiation doses or images with

enhanced CNR

• Photon counting detectors will become commercially available with vary fast readout.

• Smart hospital– electronic hospital information systems

• CAD: computer-aided diagnosis