CT Dosimetry and Risk Benefit Considerations

Question 1

what is the index of CT dose

Answer 1

CTDI vo

Question 2

what is the dose lenth product equal to

Answer 2

scan length x CTDI vol

Question 3

what are CTDivol and DLP measures of

Answer 3

CT scanner output

Question 4

what is a CTDI and draw it

Answer 4

it is the integral of the dose profile from a single axial scan- draw it

Question 5

what is the typical equipment used to measure CTDI100

Answer 5

acrylic phantoms are used for it. 32 cm diameter are for body while 16 cm diametere is used for the head.

Question 6

what is CTDIw and how do you calculate it

Answer 6

it is weight average of center and periphery doses.

=2/3CTDI1oo (edge)+1/3CTDI100 (center)

Question 7

how do you calcualte CTDI vol

Answer 7

1/pith x(CTDIw

Question 8

how do you find pitch

Answer 8

it is pitch= table index per rotation/TOTAL nominal scan width

Question 9

what dose CTDI vol represent

Answer 9

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

Question 10

is there multiple CTDI phantoms for different types of patients

Answer 10

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

Question 11

does phantom size affect the CTDI value

Answer 11

it does affect it overall

Question 12

what are diagnostic reference levels

Answer 12

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

Question 13

who regulates CT

Answer 13

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.

Question 14

what are the main reference doses for the head, abdomen, pediatric

Answer 14

adult head: 57 mgy
adult abdomen- 20mgy
pediatric abdomen- 5.6mgy

Question 15

what are the maximum doses allowed for head, abdomen, pediatric

Answer 15

adult head- 80mgy
abdomen- 30mgy
pediatric 25 mgy

Question 16

is CTDIvol equal to patient dose

Answer 16

no it is not the same. CTDI quantifies scanner radiation output. Patient size must be considered to esitmate power

Question 17

does scanner output change with different size patients

Answer 17

no it is the same output but the relative dose is different

Question 18

how do you calculate SSDE and what does SSDE stand for

Answer 18

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)

Question 19

how do you determine SSDE

Answer 19

pateint dimenstion such as AP thickness, lateral width are added then has some conversin factors to deal with size.

Fsize* CTDIvol

Question 20

is SSDE good to estimate the organ dose

Answer 20

if the organ is reasonably large and fully contained within a scan region, then SSDE is a reasonable estimate of organ dose

Question 21

what is the effective dose and how do you calculate it

Answer 21

a calculated quantity that reflexts the radiation detriment of a non-uniform exposure in terms of an equivalent whole body exposure

Question 22

how do you calculate effective dose

Answer 22

metod 1 is based on organ dose estimates, and method 2 is based on dose length product

Question 23

does tissue weighting come into the effective dose

Answer 23

it is using weighted dose

Question 24

what is DLP calculation

Answer 24

it is CTDIvol times CT scan length

effective dose is then k times DLP.

Question 25

how do you find k

Answer 25

it is five values based on regions

Question 26

what are the k regions

Answer 26

head, neck, chest, abdomen and pelvis, and head

Question 27

what is the safe range for CT

Answer 27

it is around 100 msv

Question 28

what makes low dose CT good and bad

Answer 28

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

Question 29

how is best to maximize the benefit to risk ration

Answer 29

eliminate unnecessary exams/phases. Image gently and wisely. Invest is dose reduction technology. Need to deide whether or not the exam is justified.