Notes
Radioactive Iodine U[take
Iodide entering the circulation is removed from it by active transport into the thyroid cells and also by elimination from the body in the urine.
if minimal/insufficient iodide enters bodyover long period, conservation maximized and circulatory inroganic iodide taken up by thyroid
--> tests of thyroid function using radioactive iodine as tracer must be interpreted in relation to available dietary iodine
- greater proportion of tracer taken up by thyroid in cases of iodine deficiency than will be absorbed in prescence of normal intake
- if excessive iodine ingested (medication, or dietary idiosyncrasies) only small proportion used by thyroid, remainder eliminated by kidneys
- added radioactive tracer reflects this, normal values of tracer tests shouldn't be uncritically used for interpretation of particular results
- active transport of iodide from blood into thyroid is trapping
- since avidity of thyroid iodide trap related to degree of repletion of gland and need for element, activity of trap is close indicator of metabolic activity of gland as whole
- many way so to measure radiation emanating from gland after adminstering radioactive iodine used to test activity of iodide trap (long-established thyroid uptake tests)
- thyroid doesn't distinguish b/w different isotopes of idodine so choice of tracer dictated by physical characteristics
-iodine 131 is most commoly used in dose of few mu Ci for uptake measurements and 10 to 20 mu ci is imaging of thyroid
-radiation dose to thryoid reduced byusing 2.3 hour half life , i 132 but short-half life limits measurements ot those of early uptake
-high gamma ray energyleads to resolution problems in imagining
-i 124 is reducedose but has gamma ray w/ such low energy that absorbs within gland itself and in neck tissues leading to inaccurate results
-tracer ioide given orally, amount of radioactivity in thryoid measured @ various times after dose, and most useful single time being after 24 h, but best to record in 4 h
-at earlier time, i.e. 1 h after dose, chosen for early uptake estimation
-early measurements refelct avidity of thyroid trap and 24 h measurement gives indication of thyroidal ability to incoroprate idoide into hormone precursors (organification)
-radioactivity measured @ selected time related to dose administered by apporiate calibration and recorded as %
- great overactivity ofgland, after 24 h, tracer not only organified, but secreted back into circulation incoropated into thyroid hormones
-in these cases, uptake record @ 24 h below max. and even within normal
-extra info. given by early uptake value avoid misaleding interpretiation of study
- influence of iodide repletion on uptake values , normal range must be quoted w/ caution, but 24h value in range 25%- 45% of adminstered dose normal in most populations
-relative iodien deficinecy, values higher, and in population living on diet rich in seaweed, value are lower
- high uptake value consistenc w/ hyperthyroidism and low value w/ hypothyroiidism
DOSE LIMITS IN GENERAL
-international commission on radiological protection ICRP has watching brief on protection against ionizing radiation
- publishes recommendatinos in light of accumulating experience
-dose limitation to ensure radiation related to benefits and keep necessary exposure as small as possible
-specified limits of dose shouldn't be exceeded
-
ANNUAL LIMIT OF INTAKE
-if person occupationally exposed to ionizing radiation, have neglible exposure to external irradiation
-risk mostly related to radiation emitted by ignested radionuclides
-person working in nuclear medicine subject to irradiation andruns risk of possibly ingesting radioactive materials
-amount called ALI annual limit f intake
-in estimating LAI biological behaviour of material as well as physical characteristics of radiation take into account
-chemical form of material greatly used to influence biological half-life of material in body
- physical form (colloidal, particulate, etc) also have predominating influence in calculating ALI in relation to specific nucldie
Iodide entering the circulation is removed from it by active transport into the thyroid cells and also by elimination from the body in the urine.
if minimal/insufficient iodide enters bodyover long period, conservation maximized and circulatory inroganic iodide taken up by thyroid
--> tests of thyroid function using radioactive iodine as tracer must be interpreted in relation to available dietary iodine
- greater proportion of tracer taken up by thyroid in cases of iodine deficiency than will be absorbed in prescence of normal intake
- if excessive iodine ingested (medication, or dietary idiosyncrasies) only small proportion used by thyroid, remainder eliminated by kidneys
- added radioactive tracer reflects this, normal values of tracer tests shouldn't be uncritically used for interpretation of particular results
- active transport of iodide from blood into thyroid is trapping
- since avidity of thyroid iodide trap related to degree of repletion of gland and need for element, activity of trap is close indicator of metabolic activity of gland as whole
- many way so to measure radiation emanating from gland after adminstering radioactive iodine used to test activity of iodide trap (long-established thyroid uptake tests)
- thyroid doesn't distinguish b/w different isotopes of idodine so choice of tracer dictated by physical characteristics
-iodine 131 is most commoly used in dose of few mu Ci for uptake measurements and 10 to 20 mu ci is imaging of thyroid
-radiation dose to thryoid reduced byusing 2.3 hour half life , i 132 but short-half life limits measurements ot those of early uptake
-high gamma ray energyleads to resolution problems in imagining
-i 124 is reducedose but has gamma ray w/ such low energy that absorbs within gland itself and in neck tissues leading to inaccurate results
-tracer ioide given orally, amount of radioactivity in thryoid measured @ various times after dose, and most useful single time being after 24 h, but best to record in 4 h
-at earlier time, i.e. 1 h after dose, chosen for early uptake estimation
-early measurements refelct avidity of thyroid trap and 24 h measurement gives indication of thyroidal ability to incoroprate idoide into hormone precursors (organification)
-radioactivity measured @ selected time related to dose administered by apporiate calibration and recorded as %
- great overactivity ofgland, after 24 h, tracer not only organified, but secreted back into circulation incoropated into thyroid hormones
-in these cases, uptake record @ 24 h below max. and even within normal
-extra info. given by early uptake value avoid misaleding interpretiation of study
- influence of iodide repletion on uptake values , normal range must be quoted w/ caution, but 24h value in range 25%- 45% of adminstered dose normal in most populations
-relative iodien deficinecy, values higher, and in population living on diet rich in seaweed, value are lower
- high uptake value consistenc w/ hyperthyroidism and low value w/ hypothyroiidism
DOSE LIMITS IN GENERAL
-international commission on radiological protection ICRP has watching brief on protection against ionizing radiation
- publishes recommendatinos in light of accumulating experience
-dose limitation to ensure radiation related to benefits and keep necessary exposure as small as possible
-specified limits of dose shouldn't be exceeded
-
ANNUAL LIMIT OF INTAKE
-if person occupationally exposed to ionizing radiation, have neglible exposure to external irradiation
-risk mostly related to radiation emitted by ignested radionuclides
-person working in nuclear medicine subject to irradiation andruns risk of possibly ingesting radioactive materials
-amount called ALI annual limit f intake
-in estimating LAI biological behaviour of material as well as physical characteristics of radiation take into account
-chemical form of material greatly used to influence biological half-life of material in body
- physical form (colloidal, particulate, etc) also have predominating influence in calculating ALI in relation to specific nucldie