Chapter 12 CT II Flashcards
typical x-ray tube voltage in head CT
120 kV
why use higher voltage (140kV) in posterior fossa
minimize beam hardening artefacts
why use lower tube voltage (100 kV) in angiographic imaging
maximize visibility of iodinated contrast media
why are x-ray beam intensities in head CT relatively high?
- reduce mottle
- CTDIvol 60 mGy
- can reduce with iterative reconstruction
- if imaging airways and bony structures, don’t need such high intensity as noise is not as important
dose-length-product for routine head CT exam
1000 mGy- cm
typical tube voltage in abdo and pelvic CT
120 kV
typical CTDIvol for routine abdo or pelvic scan
15 mGy
20 mGy for detecting soft tissue lesions (ex liver)
dose-length product for abdominal or pelvic scan
300 mGy-cm
CTDIvol for chest
10 mGy
tube voltages for chest contrast studies
100 kV so you can see the iodinated material
dose-length product for typical chest CT
300 mGy-cm
head normal and failing CTDIvol
60 mGy
80 mGy
abdo normal and failing CTDIvol
15-20 mGy
30 mGy
ped abdo normal and failing CTDI vol
8 mGy
> 25 mGy
as measured in small phantom
what is used for cardiac imaging
> 64 slice multi0detector CT
-need good spatial and temporal resolution
kV for cardiac imaging
80 or 100 kV because of iodinated contrast
retrospective vs prospective gating
-cardiac imaging
prospective- only acquire images at certain parts of cardiac cycle
- reduces patient dose
- retrospective gating in cardiac imaging has pitch of 0.2-0.3
how many slices multi detector CT scanner can capture heart in single rotation
250-320 slice
-also dual-source CT operates at > 3 pitch and can catch whole cardiac cycle in single heart beat
sensitivity of cardiac CT
> 90%
negative result rules out coronary heart disease
diameter of a head size phantom
18 cm
diameter of adult abdomen phantom
28 cm
diameter of adult chest phantom
24 cm
what must be kept constant at CT detector?
Kair
-modify output to keep Kair constant
kV for non-contrast studies in infants
80 kV
kV for angio exams in kids
80 kV
100 kV if cannot penetrate
acceptable mottle in pediatric CT vs adult CT
pediatric is lower than adult
how is unecessary exposure limited in pediatric CT?
limit scan lenght
how is unecessary exposure limited in pediatric CT?
limit scan lenght
-single phase exam is sufficient for most clinical tasks
how are projection radiographs (localizer scans) obtained?
- advance the patient couch through the gantry with the tube and detector in a fixed position using a fixed low mA
- use same kV as in all subsequent scans
- ensure patient is correctly centered
how is projection radiograph attenuation data used?
-used to select how the AEC will modulate the mA
how is projection radiograph attenuation data used?
- used to select how the AEC will modulate the mA
- 2 projection radiographs will improve AEC peformance
when do we select scan length and type?
after projection radiography (localizer) scan
how much do the localizer scans increase patient dose by?
a few percent
what is dual source CT?
- uses 2 x-ray tubes and 2 detector arrays
- both angular systems are mounted on a rotating gantry with angular offset of 90 degrees
- acquires twice as many CT projections
- enables high pitch - can get faster scans for patients who can’t hold their breath or can’t stay still
- image quality in central region where 2 FOVs overlap is superior to image quality in peripheral region
dual energy CT
- need 2 x-ray tubes
- uses 80 kV and 140 kV
- can acqauire similar projections at 2 energies
- improves delineation of different materials that have similar attenuation coefficients
applications of dual energy CT
- subtraction of bone
- assessment of pulmonary perfusion
- kidney stone characterization
- differentiating uric acid deposits from calcium in gout
how is dual energy CT used for liver scans?
enables iodine removal so you can generate unenhanced images
-DECT can be obtained without additional radiation dose, by acquiring high kV and low kV images with the summed dose sequal to the dose of single energy CT
describe CT fluoro
display of constantly updated images produced by continuous rotation of x-ray tube
rate of image update in CT fluoro
6 images/s
-fluoro and ultrasound update at 30 fps
kV and mA for CT fluoro
120 kV
20-50 mA - helps to minimize radiation doses
what makes up the dental cone beam CT
flat panel detectors combined with x-ray tube
scan times for dental CBCT
20 s scan times
3 min to reconstruct an image
tube voltages and currents for dental CBCT
90 kV
7 mA
focal spot size dental CBCT
0.5 mm
dental CBCT pixel size and spatial resolution
170 um pixels
uses small focal spot (0.5 mm)
spatial resoltuon 2.5 lp/mm
patient doses in dental CBCT
very low (< 0.1 mSv) or low (0.1 - 1 mSv)
what is major determinant of patient dose in dental CBCT?
irradiation geometry defined by the area of the 2D detector array
-doubling the detector array dimension will likely quadruple the patient dose as the exposed area is 4 times higher
How do changes in CT contrast with kV change depending on properties of the tissue?
-changes are small for soft tissues (Z = 7.5) but large for high Z (iodine, Z= 53)
definition of CT subject contrast
difference in HU of a lesion relative to background tissues
contrast of blood vessel with dilute iodine (HU of 180) next to tissue (HU of 50)
contrast is 130 HU
diluted iodine attenuates 13 % more than soft tissue
noise is +/- 3 HU- what are the pixel values
68 % have HU within +/- 3
95 % have HU within +/- 6
99% have HU within +/- 9
what does CT mottle depend on?
number of x-ray photons used to make the image
-proportional to mA, x-ray tube rotation time, and reconstructed slice thickness
- quadrupling mA will halve resultant mottle
- quadrupling scan time or slice thickness also halves mottle
how does reconstruction filter affect CT mottle?
filters with good resolution increase mottle
use of iterative reconstruction generally reduces the amount of mottle in CT images but also changes noise texture
currently, what is shortest time that a standard CT imager can acquire 1,000 projections during a 360-degree x-ray tube rotation
0.3 s
describe rebinning
after x-ray tube rotates 0-180 degrees, it is possible to synthesize projections pertainting to x-ray tube angles 180-360 degrees
-enales a CT image to be generated in half the x-ray tube rotation, but reduced spatial res due to reduced sampling and increased noise because fewer x-rays could create the image
best achievable temporal resolution on CT scanner
half the tube rotation time
-150 ms
dual source CT scanners acquire image data at double the rate of single source scanners, offering better temporal resolution performance (best achievable is 75 ms)
what determines spatial resolution in standard CT scanners
- focal spot size and detector size determine in plane spatial resolution (~ 0.7 lp/mm)
- detector thickness determines resolution in longitudinal direction
-inferior to digital x-ray radiogprahy (3 lp/mm) and mammo (7 lp/mm)
what can help minimize motion blur?
reduce x-ray rotation time
what reconstruction filters yield the best resolution?
detail (bone)
what does zoom reconstruction do?
uses acquired projections to reconstruct a smaller FOV to improve resolution in central image region
can smaller pixels improve CT resolution?
not if limits imposed by focal spot blur and detector blur have been reached
what is partial volume artefact?
result from averaging the linear attenuation coefficient in a voxel that is heterogeneous in composition
what is photon starvation
metal implants and long pathlengths through highly attenuating material can result in the CT detectors recording no transmission
-photon starvation yields star and streak artifacts (positive and negative) along rays through high density structures
what are ways to reduce streak artifacts
scan patient with arms above their head
iterative reconstruction
what is beam hardening artifact
low-energy photons are preferentially absorbed, resulting in more penetrating beam
- when the beam hardens, the computed HU values are lower, showing darker areas behind strong absorbers like bone
- software algorithms that incorporate prior knowledge of the patient can reduce beam hardening artifacts
single ring artifact
happens when single detector is faulty
multiple ring artifact
happens when CT scaner not properly calibrated
aliasing artifact
inadequate data sampling
edges are imaged into central FOV
cone beam artifacts
occur with large detectors due to the inadequate anatomical coverage related to undersampling in the cone beam direction
determinants of organ dose in CT
- amount of radiation used to perform CT exam
- scan length
- patient size
why is thyroid dose higher than brain dose for a head scan?
smaller size of neck and absence of attenuation by the skull
at fixed CTDIvol, organ doses to smaller vs larger patients
smaller patient = doses 20% higher
larger patient = doses 20 % smaller
is organ dose to organs that receive scattered radiation only concerning?
no
average risks from body CT scans (effective dose 5 mSv)
0.06% in 25 yo female and 0.04% in 25 yo male (remember 10%/Sv)
principal organs that contribute to cancer induction from CT scans
colon stomach lung bladder prostate
types of cancer that make up cancer risk from CT
- lung cancer
- leukemia
- breast cancer
what are k-factors
conversion factors to convert dose-length-products to effective doses in CT
-depend on anatomy, patient age and size, independent of kV
- also depend on phantoms in which CTDIvol are measured
- when a large phantom replaces a small phantom, values of CTDIvol and DLP are halved and k-factors are doubled
effective dose for head scan
2 mSv
effective dose for chest scan
5 mSv
effective dose for abdo or pelvis scan
5 mSv
why is k-factor smaller for head vs chest?
CTDI is larger for head than chest, but chest has more radiosensitive organs and thus has higher effective dose
what happens to k-factors as age is reduced for children?
increase as effective dose increases
-increases from 5X that of adults to 1.6X that of adults over 0 to 10 years old
k-factor for adult heaed, chest, abdo/pelvis/whole body (uSv/mGy-cm)
2.4, 20, 15
infant effective doses for head CT vs adults
-twice those of adults, despite using half the radiation
infant effective doses for chest CT vs adults
-a third of that of adults despite using 10X less radiation
when are embryo/fetal doses important?
when x-ray beam directly irradiates the fetus
what are embryo doses proportional to?
CTDIvol
-must always account for scan length and patient size
what is dose to embryo for multi-phase body CT scan?
sum of doses associated with each phase
embryo dose to pregnant patient during chest CT
- 1 mGy
- primarily due to internal scatter- placing lead apron wouldn’t have much effect on embryo dose
tissue HVL in CT
4 cm
will increasing kV decrease mottle?
yes because more penetration
how to decrease photon starvation?
increase kV
for a CAP CT scan with CTDIvol of 10 mGy, how much dose do we expect the organs within to get?
15 mGy
for a body scan with CTDIvol of 20 mGy, how much dose do directly irradiated organs get?
30 mGy
eye lens dose, thyroid dose, brain dose for head scan with CTDIvol 60 mGy
55, 65, 50 mGy
