CT Dosimetry and Risk Benefit Considerations Flashcards
what is the index of CT dose
CTDI vo
what is the dose lenth product equal to
scan length x CTDI vol
what are CTDivol and DLP measures of
CT scanner output
what is a CTDI and draw it
it is the integral of the dose profile from a single axial scan- draw it
what is the typical equipment used to measure CTDI100
acrylic phantoms are used for it. 32 cm diameter are for body while 16 cm diametere is used for the head.
what is CTDIw and how do you calculate it
it is weight average of center and periphery doses.
=2/3CTDI1oo (edge)+1/3CTDI100 (center)
how do you calcualte CTDI vol
1/pith x(CTDIw
how do you find pitch
it is pitch= table index per rotation/TOTAL nominal scan width
what dose CTDI vol represent
9t re-resejts tue d0se 9j tue cejtrap reg90j 0f a k8pt9-pe scaj exak t0 a su0rt cylindrical acrylic phantom for beam widths< approximately 8cm
is there multiple CTDI phantoms for different types of patients
it is the same phatom, but same Kvp and colimation, but the MAS changes, so this and the scan lenth can changes overall leading to changes. IF you have it for the adult, you can look at it and find a ratio
does phantom size affect the CTDI value
it does affect it overall
what are diagnostic reference levels
DRLs are a form of investigation level used as a simple test to identify siutations where pateint dose is unusally high. It emplys an easily measured and standardized quantity ( not effective dose). If consistently exceeded, a local review of procedures and equipment should be performed. IF possible, dose reduction measures should be taken
who regulates CT
ACR is the accredidation program, and it esatablishs the US CT diagnostic reference ranges. It is based on CTDI vol to include the effect of pitch.
what are the main reference doses for the head, abdomen, pediatric
adult head: 57 mgy
adult abdomen- 20mgy
pediatric abdomen- 5.6mgy
what are the maximum doses allowed for head, abdomen, pediatric
adult head- 80mgy
abdomen- 30mgy
pediatric 25 mgy
is CTDIvol equal to patient dose
no it is not the same. CTDI quantifies scanner radiation output. Patient size must be considered to esitmate power
does scanner output change with different size patients
no it is the same output but the relative dose is different
how do you calculate SSDE and what does SSDE stand for
it stands for size specific dose estimates, which are usedf ofr pediatrics. SSDE=CTDIvol x conversion factor.
conversion factor is exponentially related to patient size (attenutation)
how do you determine SSDE
pateint dimenstion such as AP thickness, lateral width are added then has some conversin factors to deal with size.
Fsize* CTDIvol
is SSDE good to estimate the organ dose
if the organ is reasonably large and fully contained within a scan region, then SSDE is a reasonable estimate of organ dose
what is the effective dose and how do you calculate it
a calculated quantity that reflexts the radiation detriment of a non-uniform exposure in terms of an equivalent whole body exposure
how do you calculate effective dose
metod 1 is based on organ dose estimates, and method 2 is based on dose length product
does tissue weighting come into the effective dose
it is using weighted dose
what is DLP calculation
it is CTDIvol times CT scan length
effective dose is then k times DLP.
how do you find k
it is five values based on regions
what are the k regions
head, neck, chest, abdomen and pelvis, and head
what is the safe range for CT
it is around 100 msv
what makes low dose CT good and bad
it is go to for general screening, but it can also be detrimental because it can miss thigns like pancreatic cancer because low dose is only single phase
how is best to maximize the benefit to risk ration
eliminate unnecessary exams/phases. Image gently and wisely. Invest is dose reduction technology. Need to deide whether or not the exam is justified.
