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Dual Energy CT
• Uses two different x-ray tubes in a single CT unit
• Exposing the patient with two different energy spectrums
• Development of Dual energy CT include tissue differentiation
and visualization of tendons and ligaments.
• Dual Source CT uses two rotating tubes to acquire both high
and low voltage images.
• There is significant difference of CT number due to different
attenuation coefficient at different energies.
• The two basis materials should be sufficiently different in
their atomic number Z to ensure their different photoelectric
and Compton attenuation characteristics.
• Images are dependent on the attenuation of the x-ray beam,
which depends on the voltage applied across the tube, each
image acquired is energy dependent
X-Ray Tube
• Maximum and minimum
voltage that can be applied
across the tube is 140 kVp and
80 kVp
• Largest energy difference
between the two tubes would
be 60 kVp.
• The average energies of the two
spectrums are 76 keV and 56
keV
• A tin filter may be placed in the
path of the beam to remove the
low energy x-rays(beam
hardening)
• Attenuation at moderate energy
levels is primarily due to
Compton scatter, but at lower
energy the photoelectric effect
is dominant.
Schematic diagram of a Dual Source CT unit
Application
• Certain elements within the body, such as calcium or magnesium,
have different attenuation properties and can be visualized when
images are acquired at lower tube voltages.
• Angiography in which iodine is used as a contrast agent for
vascular enhancement.
• Removal of bony objects for better visualization of the vascular
structures,
• Identification and removal of calcified plaques to investigate the
integrity of the vessels.
• Separation of benign and malignant tumors based on the iodine
contrast uptake,
• Differentiation of various lung nodule types,
• Separation of fatty content in the liver, and
• Improved characterization of renal stones
Cardiac measurement
• Phase selective imaging of the heart with
MSCT
• ECG – correlated /ECG gated/ECG Triggered CT
• Cone beam CT
• Electron Beam CT
Types of CT Acquisition
• Cardiac Sequential CT
– Can do ECG-triggered acquisition
– After an R wave the acquisition is initiated and
performed during the predicted diastolic phase based
on the R-R interval of the previous cycles. Table then
moves and waits for next R wave
– Image quality of triggered multi-slice CT regarded
inferior compared to spiral, but the radiation is used
more efficiently and thus often used for calcium
scoring in the preventative setting
Types of CT Acquisition
• Electron Beam CT (EBCT)
– A non-mechanical sequential CT scanner
– No mechanically rotating parts
– Temporal resolution is 100ms
– Acquisition time is about 50ms
– Can be triggered by ECG
– Can get a single or up to 3 acquisitions of a single
slice – thus can chose the best data set
• Multislice CT
– Instead of a single detector row, have several parallel
detector rows
– Allows simultaneous acquisition of several slices
– Again, larger sections scanned in shorter time
Types of CT Acquisition
• Spiral CT
– Table moves at a constant speed while the detectors
continuously acquire data
– Larger sections can thus be scanned in same amount
of time
– Slip ring technology made this possible as cord wrap
around is not an issue
– For rotation of 0.4s
• heart frequency =60bpm,
• maximum pitch=0.4
• Maximum table speed = collimation width/(1/heart
frequency)
ECG Gated Reconstruction
• Imaging the heart requires acquisition that is
synchronized to the motion of the heart
• Sequential scanners acquire slices prospectively triggered
by the patient ECG
• Spirals acquire continuous, overlapping data throughout
the cardiac cycle
• The recorded ECG is used afterwards to select spiral data
acquired during the same cardiac phase to reconstruct
slices
• Reconstruction of any cardiac phase can be performed
Patients with Fast Heart Rates
• Use oral or IV beta blockers to achieve slower HR unless
contraindicated
• If unable to use BB
– Alternative scanning & reconstruction methods are
available
– The effectiveness of these alternative algorithms
varies with the actual HR in relation to the rotation
time of the scanner