Chụp cắt lớp vi tính hay chụp cắt lớp CT

acquires data while table is moving Formation of a CT image - Tomographic Acquisition Formation of a CT image - Tomographic Acquisition is called a projectionor view Formation of a CT im

Computed Tomography – Chapter 13

Kalpana Kanal, Ph.D., DABRLecturer, Diagnostic PhysicsDept of Radiology

UW Medicine

a copy of this lecture may be found at:

http://courses.washington.edu/radxphys/PhysicsCourse04-05.html

Basic Principles - Conventional Radiograph

c.f Bushberg, et al The Essential Physics of Medical

Figure from Dr Mahesh, John Hopkins, MD, AAPM Handout c.f http://www.impactscan.org/slides/impactcourse/1_2_basicprinciples/img5.htm

c.f Wolbarst,The Physics of Radiology, 2nd Edition, 2005

Historical Development of CT

Rotate/Translate, Pencil Beam

the first CT scanner introduced into clinical practice

images

c.f Wolbarst,The Physics of Radiology, 2nd ed., p 409.

c.f Bushberg, et al The Essential Physics of Medical

Kanal 9

c.f Seeram,Computed Tomography, 2nd Ed., pg 79

acquires data while table is moving

Formation of a CT image - Tomographic Acquisition

Formation of a CT image - Tomographic Acquisition

is called a projectionor view

Formation of a CT image - Tomographic Acquisition

¬ You could have approx 800 rays taken at 1,000 different projection angles giving 800,000 transmission measurements

Kanal c.f www.sprawls.org, computed tomography lecture 13

Formation of a CT image - Tomographic Acquisition

patient is lying on is moved slightly in the cranial-caudal direction or the z-axis of the scanner

beam for the acquisition of the next image

Formation of a CT image – Reconstruction

Tomographic Reconstruction Preprocessing & Raw Data

¬ It= I0e-µt

¬ ln (I0/ It) = µt for each ray

reconstruction

Tomographic Reconstruction

used to produce the CT image (attenuation coefficient map)

by essentially reversing the acquisition steps

smeared along the same path in the image of the patient

attenuation reinforce each other building up the image in the computer

using a mathematical filter, or kernel

convolution technique

Image Reconstruction

with increased noise

which decreases image noise but also decreases spatial resolution

use with the reconstruction algorithm depends on the clinical task

c.f Seeram,Computed Tomography, 2nd Ed., pg.108

basic 2D element of the digital image

information concerning the patient’s anatomy that is located in the corresponding voxel (volume element)

equal to the voxel width and height

dimension that represents the slice thickness of the CT scanc.f

http://www.impactscan.org/slides/impactcourse/1_2_basicprinciples/img21.htm

field of view (FOV).

pixel is 12-bits (0-4095)

with a larger matrix (smaller pixels) or smaller FOVc.f

http://www.impactscan.org/slides/impactcourse/1_2_basicprinciples/img21.htm

c.f http://www.impactscan.org/

What Is Being Measured?

results in a 2D matrix of floating point numbers in the computer which range from near 0.0 up to value equal to 1.0

average linear attenuation coefficient of the tissue contained

in each voxel

and truncated to integer values that encompass 4096 values, between -1000 and 3095 (typically)

attenuation coefficients

CT Numbers or Hounsfield Units

¬ µ(x, y) is the attenuation coefficient for the voxel, µwateris the attenuation coefficient of water and CT (x,y) is the CT number (or Hounsfield unit) that comprises the final clinical CT image

white matter = 30, gray matter = 40, muscle = 50, dense bone andareas filled with high contrast agent range up to +3000

the physical properties of tissue that influence Compton scatter

tissue and plays the dominant role in forming contrast in medical CT

amount of calcification can be determined from the mean CT number of the nodule

be used to accurately access tumor volume or lesion diameter

Kanal 25

c.f Bushberg, et al The Essential Physics of Medical

Imaging, 2 nd ed., p 359.

Digital Image Display: Window/Level

imagers for printing have about 8 bits of display fidelity (28=256)

reduced to 8 bits to accommodate most image display hardware

determines the contrast of the image, with narrower windows resulting in greater contrast

the center of the window

c.f Seeram,Computed Tomography, 2nd Ed., pg 82

Technological Advances That Led To:

Helical (Spiral) Acquisition

Kanal 29

c.f Seeram,Computed Tomography, 2nd Ed., pg 218

Interpolation (Helical)

a helical trajectory around the patient

assume that the x-ray source has negotiated a circular, not a helical path around the patient

differences, the helical data set

is interpolated into a series of planar image data sets

additional overlapping images with no additional dose to the patient (interleaved

Pitch = Table increment per rotation (mm)

p 261.

Slice Thickness: Single Detector Array Scanners

Slice Sensitivity Profile

slice CT is determined by the physical collimation of the incident x-ray beam with two lead jaws

a section is imaged and to what extent details within the section contribute to the signal

in the center of the CT slice

X it produces greater contrast from background (greater difference in CT number) than

X when the same object is positioned near the edge of the slice volume c.f Bushberg, et al The Essential Physics of

Medical Imaging, 2 nd ed., p

343.

Kanal 33

Slice Sensitivity Profile for Conventional and Helical CT

Detectors and Detector Arrays

¬ Most modern CT systems use either Xenon detectors (old technology) or solid-state scintillator detectors

c.f Bushberg, et al The Essential Physics of Medical Imaging, 2 nd ed., p

339 & 340.

Take Home Points

image

of the CT scan

narrower windows resulting in greater contrast

Kanal 37

Raphex 2001 Diagnostic Questions

operator affects:

but has no affect on dose A smooth algorithm decreases noise and resolution A sharp algorithm increases noise and resolution

Raphex 2000 Diagnostic Questions

mm to 2 mm, the CT number of the nodule would probably:

¬ A If properly centered in the slice, the CT number will go up

because of less volume averaging with air in adjacent lung

Raphex 2003 Diagnostic Questions

¬ D45 If a CT bone window is set at a width of 1000 with the center at

500, the range of CT numbers that will be displayed as black is _

Raphex 2003 Diagnostic Questions

¬ D43 The CT value of white matter is 40 HU, and that of gray matter

is 45 HU The approximate subject contrast between white and gray matter is _

ed., p 258.

Slice Thickness: Multiple Detector Array Scanners

detector array CT scanners is determined by the width of the detectors in the slice thickness dimension

changed by binning different number of individual elements together –that is, the electronic signals generated by adjacent detector elements are electronically summed

c.f Bushberg, et al The Essential Physics

of Medical Imaging, 2 nd ed., p 344.

Slice Thickness: Multiple Detector Array Scanners

¬ It is typical to adjust the collimation so that the penumbra falls outside the edge detectors

to be higher, specially for small slice widths in multislice scanners

c.f Bushberg, et al The Essential Physics

of Medical Imaging, 2 nd ed., p 344.

Kanal 45

Multiple Detector Arrays

c.f Bushberg, et al The Essential Physics of Medical Imaging, 2 nd ed., p

2 nd ed., p 260.

Multislice CT

c.f Seeram Computed

Slice Width Selection

c.f http://www.impactscan.org/slides/impactcourse/1_2_spiralandmultislice/img25.htm

Kanal 49

6 to 16 Systems

c.f http://www.impactscan.org/slides/impactcourse/1_2_spiralandmultislice/img31.htm

Adaptive Mixed Fixed

Improved Spatial ResolutionIsotropic Resolution -All sides of the voxel have equal dimensions

¬ Pitchx= table travel per rotation

X-ray beam width

detector width

c.f http://www.impactscan.org/slides

Pitch in Multi-slice CT

with wide sections

reduced image artifacts)

Multiplanar Reformatting

sagittal, or oblique images from the original axial image data

but the slice thickness limits the z-axis resolution (better with MSCT)

Multiplanar Reformatting Oblique Reformatting

reformatting, except that the CT voxels in the stack are sampledalong an axis that is tilted from either the x or y planes

visualized with routine sagittal and coronal planes, and obliquereformatting can be useful in these instances

c.f Cody Radiographics, 22:1255-1268, 2002

Kanal 57

(a) Oblique reformatted view was obtained by using axial images (reconstructed from a helical acquisition) obtained at 2.5 mmsection thickness and a pitch of 1.5 A portion of a stent (large arrow) and a tumor (small arrows) are seen adjacent to the superior mesenteric artery (arrowhead)

c.f Cody Radiographics, 22:1255-1268, 2002

Oblique Reformatting

(b) On another oblique reformatted

thickness helical acquisition with pitch = 1.5, the stent (large arrow) and the tumor (small arrows) are much better visualized, as is the involvement of the superior mesenteric artery (arrowhead)

c.f Cody Radiographics, 22:1255-1268, 2002

Oblique Reformatting

Multiplanar Reformatting Advantages & Disadvantages

nerves and lesions in relation to surrounding structures

localize lesions and intraarticular bone fragments or foreign bodies

result in blurring and loss of structural detail

Three-Dimensional Image Display

Surface Rendering

voxels located on the edge of a structure are identified, usually

by intensity thresholding, and these voxels are displayed

structure are usually invisible

examining tubular structures (virtual bronchoscopy), the colon (virtual colonoscopy) and blood vessels

¬ Left: a 2D sagittal reformatted view from a CT colonography examination shows a polypoid projection into the colonic lumen (red circle) that did not extend through the colonic wall.

¬ Right: a 3D endoluminal surface-rendered image from the “fly-through” sequence provides an enlarged view of the lesion, which later proved to be an adenomatous polyp at endoscopic biopsy.

c.f Cody Radiographics, 22:1255-1268, 2002

(b) On a view from the interior of the artery, the left vertebral artery (L Vert), posterior inferior cerebellar artery (PICA), and neck of the aneurysm are all easily recognized, and their relative locations are readily appreciated.

view because of partial volume effects

if they were tissue, which can obscure protruding structures

colors, and often to be displayed with varying opacity levels (transparency)

information for surgical planning

Three-Dimensional Image Display

Volume Rendering

(a) Frontal volume-rendered image of

a patient with horseshoe kidney who was scheduled to undergo surgery for a mass in the right posterior-inferior aspect of the fused kidney (arrowheads)

The aorta is readily apparent (arrow)

(b) Another view is somewhat closer and more superior than the view seen in (a), and the kidney tissue has been rendered slightly more transparent.

The unusual vasculature pattern in this case is readily apparent, with two upper pole arteries (open arrows) and two lower pole arteries (arrowheads) originating below the inferior mesenteric artery (solid arrow).

c.f Cody Radiographics, 22:1255-1268, 2002

The kidney is shown as orange, the renal lesions are reddish, and the bones are white

Observing this case in motion (rotating right to left) helped the viewer appreciate the depth and interconnections of the anatomic structures.

(b) Another frame of the movie loop at

a different angle as (a) from this case demonstrates the renal artery structure

c.f Cody Radiographics, 22:1255-1268, 2002

Reformatting Techniques Maximum and Minimum Intensity Projections

operator through the object to the display screen, and only the relative maximum value detected along each ray path is retained by the computer

structures preferentially, and other lower-attenuation structures are not well visualized

c.f Cody Radiographics, 22:1255-1268, 2002

Reformatting Techniques Maximum and Minimum Intensity Projections

(a) Oblique MIP of a slab (cropped volume)

of the hepatic and pancreatic regions clearly demonstrates the enhancing regional vasculature and orally administered gastrointestinal contrast material.

The image noticeably shows vascular stenosis (arrows) caused by a surrounding tumor, but the tumor margins are not well seen.

(b) Minimum-intensity projection of the same oblique view emphasizes the hepatic biliary tree

The tumor margins (arrows) are more clearly delineated with this technique

A slightly different slab view volume was used

c.f Cody Radiographics, 22:1255-1268, 2002 Kanal c.f Cody Radiographics, 22:1255-1268, 2002 68

Curved Reformatting

plane

tubular structures (e.g., blood vessels) is suspected

such as the curve of the mandible as in the panoramic view above

along an implanted stent, or along the spinal cord

Kanal 69

volume-rendered image reformatted from the axial CT images demonstrates

an impacted molar (arrow)

locate the alveolar nerve canal (arrows) and define the formatting curve (gray line in c)

demonstrates the alveolar nerve canal in its entirety (gray line)

to the alveolar nerve canal reveals the impacted molar (arrow)

selects the image in the study, which is displayed by moving thecomputer mouse

such as precontrast and postcontrast images, images at differentwindow and level settings, or images from the previous and the current CT exams

computer in real time to visualize the image data as he or she interprets the case, following diagnostic clues from slice to slice

CT Fluoroscopy

tomographic images that are most commonly used for guidance during biopsies

data

provides excellent temporal resolution

time by observing the updated reconstruction

Take Home Points

reduced image artifacts)

slices in the z-axis direction

Kanal 73

Davis Notes

field of view, the limiting spatial resolution is:

per pixel The limiting spatial resolution (lp/mm) is = 1/(2x), = 1/(2 x 0.5) = 1.0 lp/mm = 10 lp/cm

Raphex 2001 Question

¬ D31 A CT in helical mode with collimation of 5 mm and pitch of 1 is compared to axial mode with 5 mm collimation The measured slicewidth will be:

¬ D Helical slice width is greater than axial for pitch 1 due to the interpolation algorithm which averages in data from more than 1 collimation width

have a higher patient dose than the axial mode:

¬ A 3

¬ C 1

represents an overlapped x-ray beam on the patient surface

B Low contrast detection

C Data acquisition rate

D Patient dose

E Image reconstruction time

Signal to Noise Ratio (SNR)

many sources

of photons used to produce the image

with increase in N)

target

Radiation Dose in CT

to x-ray projection imaging:

to a chest radiography

resolution and therefore the dose to the slice volume is higher because the techniques used are higher

Radiation Dose in CT

McNitt-Gray Multislice CT Workshop

delivers a considerable dose from scatter to adjacent tissues, outside the primary beam

c.f Bushberg, et al The Essential Physics of Medical Imaging, 2 nd ed., p 363.

Radiation Dose Multiple Scan Average Dose (MSAD)

that includes the dose attributable to scattered radiation emanating from all adjacent slices

average dose, at a particular depth from the surface, resulting from a large series of CT slices

McNitt-Gray Multislice CT

Radiation Dose Dose Measurement - CTDI

be accomplished with a single

dose index(CTDI)

using a pencil-type ionization chamber in phantoms that simulate heads (16 cm diameter acrylic) and bodies (32 cm diameter acrylic)