computed tomography - pusan national...

Computed Tomography

Ho Kyung [email protected]

Pusan National University

Introduction to Medical Engineering

Outline

• Computed tomography is different from– Laminography– Digital tomosynthesis

• Slip‐ring technology

• Bow‐tie filter

• CT number

• Helical CT

• Multislice CT (MSCT) or multidetector CT (MDCT)

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Computed tomography (CT)

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• An imaging modality that produces "cross‐sectional" images representing the x‐ray “attenuation properties” of the body– Tomo + graphy = (slice) + (to write)

x

y

z

x

z

y

Projection vs. tomograph

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Radiograph, 𝑝 𝑢, 𝑣 Tomograph, 𝑓 𝑥, 𝑦 𝜇 𝑥, 𝑦

Taken from W. A. Kalender's Text Material (2000)5

63 3563 35 2⁄ 100% 57%

1738 17341738 1734 2⁄ 100% 0.23%

Contrast

Classical tomography

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• Selecting a plane of interest by relative motion of source & detector while blurring undesired planes

• Known as many different names; planigraphy, stratigraphy, & laminography• Not true tomography

Linear tomography Axial transverse tomographyOnly line P1‐P2 stays in focuswhereas all others appear blurred

In principle, it simulates the backprojectionprocedure used in current times

• Shift and add

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Digital tomosynthesis

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• A compromise b/w radiography and CT– Requiring a low number (typically tens) of

projection images to compute 3D images with limited depth information

– Reconstructed by simple backprojection (or shift‐and‐add), FBP, or iterative method

– Available for chest imaging (commercially in 2004) and mammography (research prototype in 1999)

DBT prototype at the Univ. of Michigan

Park et al., Radiographics (2007)Dobbins & McAdams, EJR (2009) 9

10Taken from WA Kalender's Text Material (2000)

CT scanner generations

1st‐generationrotate‐translate type5 min/slice

2nd‐generationmultiple detector elements20 s/slice

3rd‐generationfan‐beam geometry0.5 s/slicemost popular

4th‐generationexpensivedifficult to scatt'd radiation

Brief history

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• Discovered x‐ray by Wilhelm Konrad Röntgen in 1985• Awarded the first Nobel prize to Röntgen in Physics in 1901• Formulated the reconstruction of a function from its projections by Johann Radon in 1917• Mathematical and experimental CT methods by A. M. Cormack in 1960s• Developed the first CT scanner (EMI) by Godfrey N. Hounsfield in 1972• Shared the Nobel prize in medicine and physiology by Hounsfield and Cormack in 1979• Developed the first whole‐body CT scanner (ACTA) by Robert S. Ledley in 1974• Developed the helical CT in 1989• Developed the multislice CT in 1998

• Even though radiation risks, CT utilization is still increasing because:– excellent image quality & rapid acquisition time

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Single‐slice Multi‐slice or multi‐detector

Overview

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X

Y

Z

X‐ray source

Detector array

Isocenter

Image reconstruction

Sinogram

Reconstructed images

System

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• Collimator– limiting the transmitted x rays to the

selected slice– preventing useless irradiation to the patient

• Post‐patient collimator or antiscatter grid– limiting the detected scattered radiation– usually, solid‐state detectors employs

external in‐plane septa

• Slip rings– to transmit power thru a brush slip ring– to transmit data via an RF or optical slip ring– sliding contacts that eliminate the

mechanical problems

• Gantry– containing the rotating parts– can be tilted over a limited angle for

imaging oblique slices

Slip ring

• Allowing the rotating gantry to have electrical connections to the stationary components• Concentric metal bands that are connected to a series of gliding contacts• Having enabled helical (spiral) CT scanning modes, & led to major reductions in CT scan

times

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Gantry & table

• Gantry is comprised of imaging hardware

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Bow‐tie filters

• Beam shaping filter designed to attenuate more toward the periphery of the field• Making signal levels at the detector more homogeneous• Reduce dose to the patient with no loss of image quality

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• No bow tie– Higher dose levels at the periphery for

cylindrical objects because of higher dose at the exit point for peripheral ray (P) than central ray (C)

• Small bow tie for a larger object– Head bow tie for a body scan– Higher dose levels at the center of objects

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Cone‐beam geometry

• Fan‐beam projection– Fan of data that converges on a vertex (i.e., x‐ray focal spot)

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• Narrow cone‐beam geometry in all modern MDCT scanners

• Wide cone‐beam geometry with flat‐panel detectors– True CB scanners w/ cone half angles

approaching 10 degrees– Image‐guided radiotherapy, dental CT,

and other breast, extremity, & SPECT/CT systems

CT image

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• Square 512 512 image matrix• Volume field of view (FOV)

– circular FOV (50 – 70 cm in diameter) over length of patient

Detectors

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• Energy integrating detectors

– Scintillation crystal with photomultiplier tube (PMT)• Scintillation crystals: NaI, CaF2, BGO …

– Converting x rays into visible light (scintillations)• PMT

– Converting light into an electric current• Pros: high quantum efficiency, fast response time• Cons: low packing density

Scintillator

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– Gas ionization chambers• Consisting of a pressurized (10 – 30 bars) gas chamber (Xenon) with electrodes• Gas ionization drifts electron‐ion pairs along field lines induction of electric currents• Pros: high packing density• Cons: low quantum efficiency (~ 60%), slow response time (~ 700 s)

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– Scintillation crystals with photodiode (most recent commercial CT detectors)• Scintillation crystals: CdWO4, Y2O3, CsI, Gd2O2S

– Individual scintillator pieces are assembled into a reflector matrix in order to define the detector cells

– Few mm thickness to have a very high absorption efficiency (96%)» Considering the finite thickness of the septa in the antiscatter grid, the absorption efficiency is

limited by the area fill fraction (~80%)– Good transfer of light to the photodiode– Very fast response time (~s)

‒ Solid‐state or semiconductor detectors (photodiodes)‒ Multichannel readout electronics or data acquisition

system (DAS)• Integrating the photocurrent from the diode and

converting the electric charge signal to voltage using a transimpedance amplifier

• Performing the analog to digital conversion with typical sampling rates (~kHz)

‒ Susceptible to electronic noise introduced by the transimpedance amplifier

• Dominant at low signal levels, leading to noise streaks in the images

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• Photon counting detectors– Based on direct conversion

• Detector materials: cadmium telluride (CdTe) or cadmium zinc telluride (CdZnTe or CZT)• Converting x‐ray photon into a certain electronic charge proportional to its energy

– x10 larger than that produced by the scintillator/photodiode combination– The electronic noise no longer dominates the signal from individual x‐rays

• Electronic circuits detect charge packages and count the number of photons instead of integrating their energy

– Improving the CNR by 10 to 20%• Remaining challenges include stability and the count rate limits

Image courtesy: E. Roessl et al., Philips (2009)

28M. J. Willemink et al., Radiology (2018)

31C. H. McCollough et al., Radiology (2015)

32C. H. McCollough et al., Radiology (2015)

Image quality

34Taken from W. A. Kalender's Text Material (2000)

80 80 pixels (w/ slice 13 mm)

1024 1024 pixels

CT number

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• CT measures and computes the spatial distribution of the linear attenuation coefficient 𝜇 𝑥, 𝑦– Note that 𝜇 𝑥, 𝑦 ~𝑓 𝐸, 𝑍– Impossible in direct comparison of images obtained CT systems with different voltages and

filtration

Instead, use CT value as a so called “CT number” in Hounsfield unit (HU)• Compute attenuation coefficient relative to the

attenuation of water• Hounsfield units (HU) in the range of ‐1000 to 1000

‒ CT number in HU 1000

• Air = ‐1000• Water = 0• Bone = the positive side scale but no unique CT

number ( 𝜇 ~ composition, structure, 𝐸)• Immune to different spectra

HU

-1000

waterair 0 T

CT#

• CT number differences due to effective atomic numbers decrease for higher energies• Contrast at high energies is dominated by density differences• Negative contrast of fat is caused both by the low effective atomic number and by the low

density


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• Typically the CT image consists of 512 512 pixels representing the CT numbers• Due to the large dynamic range, window/leveling must be used to view a CT image

– Choose the center and width (C/W) of the window

Taken from W. A. Kalender's Text Material (2000)

Original

Bone window Lung window

(Bimodal histogram)

Axial CT acquisition

• Circular or sequential CT scanning– Step‐and‐shoot mode of CT scanners– X‐ray beam “off” while the patient is being translated b/w acquisition cycles

• e.g., 64 detector arrays with width = 0.625 mm, table is moved a distance of 64 0.625 mm (about 40 mm) b/w acquisitions

– Scan a number of consecutive slices with respect to an entire volume– Must be satisfied the Nyquist criterion in axial sampling

• At least two slices per slice thickness to minimize aliasing from axial sampling

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Helical CT acquisition

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• Nowadays widely spread and also known as spiral CT• While the x‐ray tube rotates continuously around the patient, the patient is slowly

translated thru the gantry• The table feed (TF) = the axial distance over which the table translates during a complete

tube rotation of 360

• Pitch = the ratio b/w the TF & slice thickness ∆𝑧 (= ∆)

• Similar to circular CT, max. sampling distance = ∆𝑧/2 ⟹ max. TF = ∆𝑧/2 (pitch = 0.5) for a slice thickness ∆𝑧

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– 𝛽 = the angular position of x‐ray tube; 𝑧 = its axial position relative to the patient

• Consider a slice reconstruction at 𝑧– Only one view at 𝛽∗ is available– Solved by interpolation from measurements at adjacent axial positions

• 2𝜋 linear interpolation• 𝜋 linear interpolation using the double axial sampling for concurrent but opposite rays (parallel‐beam)

– Sampling distance = TF/2– Max. TF = ∆𝑧 (pitch = 1.0)

• Practically, a slightly larger pitch is often used (1.0 < pitch < 2.0)– e.g., ∆𝑧 = 2 mm with TF = 3 mm (pitch = 1.5)– Fast scan time by a higher pitch ⟹ lower patient dose, less tube load, less motion artifact, but poor

image quality

Sequential CT Helical CT(2𝜋 linear interpolation) (𝜋 linear interpolation)

Multi‐slice CT

• Employing multiple detector rows to measure several slices per rotation• e.g., a 64‐row systems with 0.5 mm detectors (referred to the isocenter), a pitch of 1.0 & a

0.33 s rotation ⟹ lung scan (40 cm) in ~4 s– 192 rows & 0.25‐s rotation ⟹ scan time in ~ 1 s

• Parallel‐beam reconstruction for 4‐slice scanners• Tilted plane reconstruction for 16‐slice scanners• 3D reconstruction for 64 and higher slice scanners

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• Cone‐beam CT• Entire volume (of interest) in one single scan• Required a 2D array of detector elements• Tuy’s data sufficiency condition for exact reconstruction

– X‐ray source trajectory cuts every plane through the object to be reconstructed– CB scan does not satisfy the Tuy’s condition– Conventional FBP cannot be applicable

• Grangeat’s Radon inversion– Applicable to CB measurements of short objects whose projections are not truncated

• Approximate reconstructions (FDK algorithm)– Circular trajectory for CB measurements– Extended version of the 2D weighted FBP to 3D– Unavoidable cone‐beam artifacts

Volumetric CT

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• Helical cone‐beam reconstruction– Resulting in a complete data set if the pitch is not too large– Exact FBP algorithms

• Katsevich• Derivative backprojection (DBP) approach

– Nevertheless, FDK‐based approaches are popular because they are preferable in terms of noise, noise uniformity and other quality characteristics

• Iterative reconstructions– Widely used in nuclear medicine– Computationally very expensive– Maximum‐likelihood (ML)– Maximum‐a‐posteriori probability (MAP)

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Cardiac CT

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• The full heart in a few seconds by a low‐pitch helical scan or a limited number of large‐coverage circular scans (or axial scanning)

• Reconstruction of a particular cardiac phase with the ECG gating signal– Dynamic 3D imaging (often called 4D images) by subdividing the cardiac cycle into different

phases

Dual‐energy CT

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• A better tissue characterization method• Requiring two different energy spectra measurements

– Two consecutive scans at different kV– Fast kV switching b/w low and high kV while a single circular or helical

acquisition– Dual‐source CT– Sandwich detectors– Multi‐channel photon‐counting detectors

• Basis material decomposition process– In projection domain

• Eliminating beam hardening artifacts, which causes a shift in average energy and corresponding attenuation coefficients

– In reconstructed image domain• Applications

– elimination of beam hardening artifacts– automatic segmentation– retrospective generation of monochromatic images– tissue characterization– virtual unenhanced images

Dedicated scanners

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• Circular cone‐beam scanning with flat‐panel detectors• Limited FOV and not critical scan time

• Oral and maxillofacial CT

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• Interventional CT– C‐arm systems– O‐arm systems

• Brest CT

Electron beam tomography

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• Called 5th‐generation CT, ultrafast CT, or cardiovascular CT• Temporal resolution = 17 fps (2 slices per frame)

Dynamic spatial reconstructor (DSR)

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• 4D x‐ray CT scanner developed at the Mayo Clinic by Richard Robb et al.– 240 cross sections at 60 Hz– 14 x‐ray sources + 14 2D detectors

Inverse‐geometry CT

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• Eliminating cone‐beam artifacts

Robotic trajectory

52Kalender & Kyriakou, EJR (2007)

A courtesy of Siemens Medical Systems

Future expectations

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• CT will remain the preferred modality for the visualization of the skeleton, calcifications, the lungs, and probably the gastrointestinal tract.– patients with a metallic implant, an electrical stimulator, or an artificial respirator

• An increased use can be expected for screening (heart, chest, colon), perfusion imaging and vascular and cardiac imaging.

• From a technical point of view– toward dose reduction, increased volume coverage, higher CNR, and improved spatial & temporal

resolution– new reconstruction algorithms for artifact reduction and for low dose CT– multi‐energy CT with optimal dose efficiency

Wrap‐up

• Computed tomography is different from– Laminography– Digital tomosynthesis

• Slip‐ring technology

• Bow‐tie filter

• CT number

• Helical CT

• Multislice CT (MSCT) or multidetector CT (MDCT)

54

computed tomography - pusan national...

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