9/26/200611ENGG 167 MEDICAL IMAGINGLecture 4: Sept. 28Chapter 5: X-ray prod., tubes & generators Chapter 6: Film-Screen RadiographyChapter 11: Digital RadiographyReferences: Bushberg textThe Physics of Medical Imaging, Webb, IOP Publ.2X-Ray tubesRef: Bushberg9/26/200623Bremsstrahlung energy spectrumRef: BushbergWhat does kVp mean ?4Bremsstrahlung & Characteristic radiation spectrumRef: Bushberg9/26/200635Modern x-ray tubesRef: Bushberg6Cathode filamentsRef: Bushberg9/26/200647Electron beam focusingRef: Bushberg8Anode GeometryRef: Bushberg9/26/200659Bevelled anode designRef: Webb10Modular system schematicRef: Bushberg9/26/2006611Current and voltage supplies for x-ray tubeRef: Bushberg12High frequency rectifierRef: Bushberg9/26/2006713High Frequency generator schematicRef: Bushberg14Descriptors of X-ray beamsQuality – penetrability of an x-ray beam, which is determined by the kVp, the target material and the filtration of the beam. (higher energy has higher HVL and higher penetration)Quantity – number of photons which is proportional to tube current times the exposure time(the higher the mAs, the higher the quantity)Exposure – proportional to the energy fluence (mR/min)Ref: Bushberg9/26/2006815Factors affecting X-ray emission1) Target material - affects quality and quantity of radiation emitted2) Tube voltage – kVp - determines maximum energy of photons emitted in the bremsstrahlung spectrum, thus affecting quality of the beam and the overall exposure. Rule of thumb 1 : Exposure α (kVp)2 Rule of thumb 2: for a fixed exposure (kVp1/kVp2)5= mAs2/ mAs13) Tube current (mA)4) Exposure time (s)– duration of x-ray production. Often the current and exposure time are expressed together as a product, in mAs.5) Beam filtration – modifies the x-ray energy spectrum and the overall number of photons in the beam.6) Generator waveform – affects the spectrum and quantity of photons emitted. Single phase system provides lower average energy and number of photons than does a three phase system.7) Focal spot size – affects the number of photons being produced.Ref: Bushberg16Power Rating of an X-ray systemPower = 100 kVp x Amaxfor a 0.1 second exposureRef: Bushberg9/26/2006917X-ray generator focal spot versus powerRef: Bushberg18Signal to Noise estimatesRef: Webb, chapter 2E - Photon energy ES(x,y,E,Ω) - Number of scattered photonsε(Ε,θ) – energy absorption efficiency of the receptor9/26/20061019Average ScatterRef: Webb, chapter 220ContrastRef: Webb, chapter 2I1I2Figure 2.69/26/20061121Contrast (combine eqn.s 2.2 and 2.6)Ref: Webb, chapter 2Therefore factors affecting contrast are:- Target tissue thickness- Linear attenuation coefficient difference- scatter-to-primary ratio, R.22Transmission & beam hardeningRef: Webb, chapter 29/26/20061223Noise level versus doseRef: Webb, chapter 224Noise level versus doseRef: Webb, chapter 2A rule of thumb is that humans can perceive objects when SNR is 5 or greater (ref. A. Rose, Vision: human and electronic, (Plenum: New York) 1973.)9/26/20061325Noise level versus doseRef: Webb, chapter 2(ref. A. Rose, Vision: human and electronic, (Plenum: New York) 1973.)26Noise level versus doseRef: Webb, chapter 29/26/20061427Radiography – Bushberg Chapter 6Ref: Bushberg28Half Value Layers (HVL) for “tissue” versus kVpRef: Bushberg9/26/20061529Magnification in RadiographyRef: BushbergFocal spot size, F, and edge of gradient, f, observed.f = F (M-1), where M = I/O = SID/SOD30Loss of resolution due to film screen systemRef: Bushberg9/26/20061631H & D curve - Exposure versus film optical density (OD)Ref: Bushberg32H & D curve - Exposure versus film optical density (OD)Ref: Bushberg9/26/20061733Film Screen contrast versus exposureRef: Bushberg34Dose and Contrast versus kVpRef: Bushberg9/26/20061835Scatter to primary beam ratioRef: Bushberg36Contrast versus S/P ratioRef: Bushberg9/26/20061937Antiscatter grids in radiographyRef: Bushberg38Antiscatter grids in radiographyRef: Bushberg9/26/20062039Bucky factor for antiscatter gridsRef: Bushberg40ArtifactsRef: Bushberg9/26/20062141Digital RadiographyRef: Bushberg42CCD detectors coupled to image intensifiersRef: Bushberg9/26/20062243CCD detectors coupled to image intensifiersRef: Bushberg44Flat panel detectors – latest technologyRef: Bushberg9/26/20062345Flat panel detectors – indirect versus direct detectionRef: Bushberg46Journal Club I – This weekChoose one paper published within the past few years, from one of the following journals: RadiologyMedical PhysicsPhysics in Medicine & BiologyInt. J. Radiation Oncol. Biol. Phys.1) Read the paper entirely. 2) Prepare a BRIEF summary of the paper3) Present to the rest of the class (Thurs or Friday)(absolute maximum 5-10 minutes)(maximum 2 slides/overheads).Links to all journals are on the class