Computed Tomography II

daughterduckUrban and Civil

Nov 15, 2013 (3 years and 9 months ago)

92 views

Computed Tomography II

Detectors and detector arrays

Details of acquisition

Tomographic reconstruction

Detectors


Xenon detectors


Solid
-
state detectors


Multiple detector arrays

Xenon detectors


Use high
-
pressure (about 25 atm)
nonradioactive xenon gas, in long thin cells
between two metal plates


Very thick (e.g., 6 cm) to compensate in
part for relatively low density


Thin metal septa separating individual
detectors improves geometric efficiency by
reducing dead space between detectors

Xenon detectors (cont.)


Long, thin plates are highly directional


Must be positioned in a fixed orientation
with respect to the x
-
ray source


Cannot be used for 4
th

generation scanners
because those detectors must record x
-
rays
as the source moves over a wide angle

Solid
-
state detectors


Composed of a scintillator coupled tightly
to a photodetector (typically a photodiode)


Scintillator emits visible light when an x
-
ray is absorbed, similar to an x
-
ray
intensifying screen


Photodetector converts light intensity into
an electrical signal proportional to the light
intensity

Solid
-
state detectors (cont.)


Detector size typically 1.0 x 15 mm (or 1.0
x 1.5 mm for multiple detector arrays)


Scintillators used include CdWO
4

and
yttrium and gadolinium ceramics


Better absorption efficiency than gas
detectors because of higher density and
higher effective atomic number

Solid
-
state detectors (cont.)


To reduce crosstalk between adjacent detector
elements, a small gap between detector elements is
necessary, reducing geometric efficiency
somewhat


Top surface of detector is essentially flat and
therefore capable of x
-
ray detection over a wide
range of angles


Required for 4
th

generation scanners and used in
most high
-
tier 3
rd

generation scanners as well

Multiple detector arrays


Set of several linear detector arrays, tightly
abutted


Use solid
-
state detector arrays


Slice width is determined by the detectors,
not by the collimator (although collimator
does limit the beam to the total slice
thickness)


Multiple detector arrays (cont.)


3
rd

generation multiple detector array with 16
detectors in the slice thickness dimension and 750
detectors along each array uses 12,000 individual
detector elements


4
th

generation scanner would require roughly 6
times as many detector elements; consequently
currently planned systems use 3
rd

generation
geometry

Slice thickness:

single detector array scanners


Determined by the physical collimation of the
incident x
-
ray beam with two lead jaws


Width of the detectors places an upper limit on
slice thickness


For scans performed at the same kV and mAs, the
number of detected x
-
ray photons increases
linearly

with slice thickness


Larger slice thicknesses yield better contrast
resolution (higher SNR), but the spatial resolution
in the slice thickness dimension is reduced

Slice sensitivity profile


For single detector array scanners, the shape of the
slice sensitivity profile is a consequence of:


Finite width of the x
-
ray focal spot


Penumbra of the collimator


The fact that the image is computed from a number of
projection angles encircling the patient


Other minor factors


Helical scans have a slightly broader slice
sensitivity profile due to translation of the patient
during the scan

Slice thickness:

multiple detector array scanners


In axial scanning (i.e., with no table movement)
where, for example, four detector arrays are used,
the width of the two center detector arrays almost
completely dictates the thickness of the slices


For the two slices at the edges of the scan, the
inner side of the slice is determined by the edge of
the detector, but the outer edge is determined
either by the outer edge of the detector or by the
collimator penumbra, depending on collimator
adjustment

Slice thickness: MDA (cont.)


In helical mode, each detector array contributes to
every reconstructed image


Slice sensitivity profile for each detector array needs to
be similar to reduce artifacts


Typical to adjust the collimation so that the focal
spot


collimator blade penumbra falls outside the
edge detectors


Causes radiation dose to be a bit higher (especially for
small slice widths)


Reduces artifacts by equalizing the slice sensitivity
profiles between the detector arrays

Detector pitch/collimator pitch


Pitch is a parameter that comes into play when
helical scan protocols are used


In a helical scanner with one detector array, the
pitch is determined by the collimator


Collimator pitch = table movement (mm) per 360
-
degree rotation of gantry / collimator width (mm)
at isocenter


Pitch may range from 0.75 (overscanning) to 1.5
(faster scan time, possibly smaller volume of
contrast agent)

Pitch (cont.)


For scanners with multiple detector arrays,
collimator pitch is still valid


Detector pitch = table movement (mm) per 360
-
degree rotation of gantry / detector width (mm)


For a multiple detector array scanner with
N

detector arrays, collimator pitch = detector pitch /
N


For scanners with four detector arrays, detector
pitches running from 3 to 6 are used

Tomographic reconstruction


Rays and views: the sinogram


Preprocessing the data


Interpolation (helical)

Sinogram


Display of raw data acquired for one CT slice
before reconstruction


Rays are plotted horizontally and views are shown
on the vertical axis


Objects close to the edge of the FOV produce a
sinusoid of high amplitude


Bad detector in a 3
rd

generation scanner would
show up as a vertical line on the sinogram

Rays and views


1
st

and 2
nd

generation scanners used 28,800 and
324,000 data points, respectively


State
-
of
-
the
-
art scanner may aquire about 800,000
data points


Modern 512 x 512 circular CT image contains
about 205,000 image pixels


Number of rays affects the radial component of
spatial resolution; number of views affects the
circumferential component of the resolution

Number of rays


CT images of a simulated object
reconstructed with differing numbers of
rays show that reducing the ray sampling
results in low
-
resolution, blurred images

Number of views


CT images of the simulated object
reconstructed with differing numbers of
views show the effect of using too few
angular views (
view aliasing
)


Sharp edges (high spatial frequencies)
produce radiating artifacts that become
more apparent near the periphery of the
image

Preprocessing


Calibration data determined from air scans
(performed by the technologist or service
engineer periodically) provide correction
data that are used to adjust the electronic
gain of each detector


Variation in geometric efficiencies caused
by imperfect detector alignments is also
corrected

Interpolation


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


Before the actual CT reconstruction, the helical
data set is interpolated into a series of planar
image sets


With helical scanning, CT images can be
reconstructed at any position along the length of
the scan

Interpolation (cont.)


Interleaved reconstruction allows the placement of
additional images along the patient, so that the
clinical examination is almost uniformly sensitive
to subtle abnormalities


Adds no additional dose to the patient, but
additional time is required to reconstruct the
images


Actual spatial resolution along the long axis of the
patient still dictated by slice thickness