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Re: CT Shielding
>References: <v01540b00b17d75d56534@[206.175.195.170]>
>Date: Tue, 12 May 1998 09:55:25 -0400
>Reply-To: Medical Physics Mailing List <MEDPHYS@LISTS.WAYNE.EDU>
>Sender: Medical Physics Listserver <medphys@lists.wayne.edu>
>From: "Dr. Robert Dixon" <rdixon@RAD.BGSM.EDU>
>Subject: Re: CT Shielding
>To: Multiple recipients of list MEDPHYS <MEDPHYS@LISTS.WAYNE.EDU>
>
>Not only are floors and ceilings getting thinner, but many concrete
>slabs use "light weight" concrete(115 lb/ft^3). Most are poured on a
>metal pan and have a trapezoidal thickness variation, such that the
>minimum concrete thickness may be several inches less than the maximum
>thickness. The architect will invariably quote the maximum thickness to
>the shielding designer and must be pressed to give the minimum. The
>shielding designer should use the minimum thickness, or at least the
>attenuation-weighted average thickness (which is not the same as the
>arithmetic mean thickness). A floor or ceiling slab may well have a
>minimum thickness of 3" light weight concrete which is equivalent to
>only 2.3" normal density concrete or 0.5 mm of lead.If your calculations
>call for 1/16" lead in the walls, then you'll probably need about the
>same lead equivalence in the floor and ceiling. A helical scanner in a
>busy facility will turn out about 4,000 scans(360 deg rotations) in a 40
>hr week(about 22,000 mA.min/wk). The concrete floor or ceiling thickness
>mentioned above will not be sufficient for a fully occupied area above
>or below(T=1).
>The ceiling shielding requirement will not be significantly less than
>the floor for the same occupancy factor. A CT isocenter is about 3 ft
>above the floor and the typical slab-to-slab distance is about 12 ft
>minimum. The sensitive organs of a person on the floor above will be
>about 2 ft above the floor if seated. The sensitive organs of a person
>on the floor below will be at most 6 ft above the floor. The respective
>distances are 9 ft and 11 ft for the floor and ceiling calculations-
>only about a 50% inverse square difference.
>Since the isodose curves bow in directly above the gantry(and presumably
>also directly below), one can push the limit by using a 30 degree
>obliquity factor for the ceiling thickness(x/cos30).The 20 gauge metal
>pan may also help a little( it is equivalent to about 0.5" concrete).
>
>Hope this helps.
>
>Bob Dixon
>Joel E. Gray, Ph.D. wrote:
>>
>> ** Mail from Medphys Listserver **
>>
>> I don't know who this e-mail was address to. However, in case you missed
>> my previous comments about floor and ceiling shielding I will repeat them
>> here--
>>
>> One word of caution-- If the CT scanner is used primarily for (or for a
>> high proportion of) head scans there is an additional problem to consider.
>> Since the slices are in the head there will be little absorber on the far
>> side of the gantry, i.e., away from the couch. Furthermore, the walls on
>> the far side of the scanner may be closer to the gantry. In this case you
>> should consider additional shielding in the walls and should also take a
>> close look at the ceiling and floor. Since the distance to the floor is
>> minimal (about a meter or less), the floor may receive the highest dose of
>> any barrier in the room.
>>
>> Concrete floors are becoming thinner. It may be necessary to add some lead
>> to the floor (under the floor, i.e., above the ceiling below) to assure
>> that you meet the appropriate protection levels for employees downstairs.
>>
>> There will be a paper forthcoming on this topic in Health Physics by S.
>> Langer and J. Gray.
>>
>> Joel
>>
>> Joel E. Gray, Ph.D., Consultant
>> 2804 Second Street SW, Suite 334
>> Rochester, Minnesota 55902
>>
>> Ph 507-286-8910
>> Cell Ph 507-269-4247
>> Fax 507-286-8910
>> e-mail joelgray@m3.sprynet.com
>>
>> > ** Mail from Medphys Listserver **
>> >If you reply to this message, it will be posted on Medphys for all the
>> >subscribers to review ...
>> >
>> >
>> >Joe:
>> >
>> >Do we need to consider floor and ceiling for CT shielding? There is no
>> >primary beam on these barrier. You mean the typical concrete for floor or
>> >ceiling is not thick enough to block the scatter radiation?
>> >
>> >
>> >Richard Wu
>
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