Mississippi Administrative Code
Title 15 - Mississippi Department of Health
Part 21 - Division of Radiological Health
Subpart 78 - Radiological Health
Chapter 1 - REGULATIONS FOR CONTROL OF RADIATION IN MISSISSIPPI
Subchapter 6 - X-Rays In The Healing Arts
Rule 15-21-78-1.6.9 - Computed Tomography X-Ray Systems
Universal Citation: MS Code of Rules 15-21-78-1.6.9
Current through September 24, 2024
1. Definitions. In addition to the definitions provided in 1.1.2 and 1.6.2 of these regulations, the following definitions shall be applicable to 1.6.9:
a. "Computed tomography dose index" means the
integral from -7T to +7T of the dose profile
along a line perpendicular to the tomographic plane divided by the product of
the nominal tomographic section thickness and the number of tomograms produced
in a single scan, that is:
where:
z= Position along a line perpendicular to the tomographic plane.
D(z) = Dose at position z.
T = Nominal tomographic section thickness.
n = Number of tomograms produced in a single scan.
This definition assumes that the dose profile is centered around z = 0 and that, for a multiple tomogram system, the scan increment between adjacent scans is nT.
b. "Contrast scale" means the change in the
linear attenuation coefficient per CTN relative to water, that is:
where:
µx = Linear attenuation coefficient of the material of interest.
µw = Linear attenuation coefficient of water.
(CTN)x
= CTN of the material of interest.
(CTN)w
= CTN of water.
c. "CS" (See "Contrast scale").
d. "CT conditions of operation" means all
selectable parameters governing the operation of a CT x-ray system including,
but not limited to, nominal tomographic section thickness, filtration, and the
technique factors as defined in 1.6.2.
e. "CTDI" (See "Computed tomography dose
index").
f. "CT gantry" means the
tube housing assemblies, beam-limiting devices, detectors, and the supporting
structures and frames which hold these components.
g. "CTN" (See "CT number").
h. "CT number" means the number used to
represent the x-ray attenuation associated with each elemental area of the CT
image.
where:
k= A constant31
µx = Linear attenuation coefficient of the material of interest.
µw = Linear attenuation coefficient of water.
i. "Dose profile" means the dose as a
function of position along a line.
j. "Elemental area" means the smallest area
within a tomogram for which the x-ray attenuation properties of a body are
depicted. (See also "Picture element").
k. "Multiple tomogram system" means a
computed tomography x-ray system which obtains x-ray transmission data
simultaneously during a single scan to produce more than one
tomogram.
l. "Noise" means the
standard deviation of the fluctuations in CTN expressed as a
percentage of the attenuation coefficient of water. Its estimate
(S
n) is calculated using the following
expression:
where:
CS = Contract scale.
µw = Linear attenuation coefficient of water.
s = Estimated standard deviation of the CTN of picture elements in a specified area of the CT image.
m. "Nominal
tomographic section thickness" means the full width at half-maximum of the
sensitivity profile taken at the center of the cross-sectional volume over
which x-ray transmission data are collected.
n. "Picture element" means an elemental area
of a tomogram.
o. "Reference plane"
means a plane which is displaced from and parallel to the tomographic
plane.
p. "Scan" means the complete
process of collecting x-ray transmission data for the production of a tomogram.
Data can be collected simultaneously during a single scan for the production of
one or more tomograms.
q. "Scan
increment" means the amount of relative displacement of the patient with
respect to the CT x-ray system between successive scans measured along the
direction of such displacement.
r.
"Scan sequence" means a preselected set of two or more scans performed
consecutively under preselected CT conditions of operation.
s. "Scan time" means the period of time
between the beginning and end of x-ray transmission data accumulation for a
single scan.
t. "Single tomogram
system" means a CT x-ray system which obtains x-ray transmission data during a
scan to produce a single tomogram.
u. "Tomographic plane" means that geometric
plane which is identified as corresponding to the output tomogram.
v. "Tomographic section" means the volume of
an object whose x-ray attenuation properties are imaged in a tomogram.
2 Requirement for Equipment.
a. Termination of Exposure.
i. Means shall be provided to terminate the
x-ray exposure automatically by either de-energizing the x-ray source or
shuttering the x-ray beam in the event of equipment failure affecting data
collection. Such termination shall occur within an interval that limits the
total scan time to no more than 110 percent of its preset value through the use
of either a backup timer or devices which monitor equipment function.
ii. A visible signal shall indicate when the
x-ray exposure has been terminated through the means required by
1.6.9(2)(a)(i).
iii. The operator
shall be able to terminate the x-ray exposure at any time during a scan, or
series of scans under CT x-ray system control, of greater than one-half second
duration.
b. Tomographic
Plane Indication and Alignment.
i. For any
single tomogram system, means shall be provided to permit visual determination
of the tomographic plane or a reference plan offset from the tomographic
plane.
ii. For any multiple
tomogram system, means shall be provided to permit visual determination of the
location of a reference plane. This reference plane can be offset from the
location of the tomographic planes.
iii. If a device using a light source is used
to satisfy 1.6.9(2)(b)(i) or (ii), the light source shall provide illumination
levels sufficient to permit visual determination of the location of the
tomographic plane or reference plane under ambient light conditions of up to
500 lux.
c. Beam-On and
Shutter Status Indicators and Control Switches.
i. The CT x-ray control and gantry shall
provide visual indication whenever x-rays are produced and, if applicable,
whether the shutter is open or closed.
ii. Each emergency button or switch shall be
clearly labeled as to its function.
d Indication of CT Conditions of Operation The CT x-ray system shall be designed such that the CT conditions of operation to be used during a scan or a scan sequence shall be indicated prior to the initiation of a scan or a scan sequence. On equipment having all or some of these conditions of operation at fixed values, this requirement may be met by permanent markings. Indication of CT conditions of operation shall be visible from any position from which scan initiation is possible.
e. Extraneous Radiation. When data
are not being collected for image production, the radiation adjacent to the
tube port shall not exceed that permitted by 1.6.4(3).
f Maximum Surface CTDI Identification The angular position where the maximum surface CTDI occurs shall be identified to allow for reproducible positioning of a CT dosimetry phantom.
g Additional Requirements Applicable to CT X-Ray Systems Containing a Gantry Manufactured After September 3, 1985.
i. The total error in the indicated location
of the tomographic plane or reference plane shall not exceed 5
millimeters.
ii. If the x-ray
production period is less than one-half second, the indication of x-ray
production shall be actuated for at least one-half second. Indicators at or
near the gantry shall be discernible from any point external to the patient
opening where insertion of any part of the human body into the primary beam is
possible.
iii. The deviation of
indicated scan increment versus actual increment shall not exceed plus or minus
1 millimeter with any mass from 0 to 100 kilograms resting on the support
device. The patient support device shall be incremented from a typical starting
position to the maximum incremented distance or 30 centimeters, whichever is
less, and then returned to the starting position. Measurement of actual versus
indicated scan increment may be taken anywhere along this travel.
iv. Premature termination of the x-ray
exposure by the operator shall necessitate resetting of the CT conditions of
operation prior to the initiation of another scan.
3 Facility Design Requirements.
a. Aural Communication. Provision shall be
made for two-way aural communication between the patient and the operator at
the control panel.
b Viewing Systems.
i. Windows, mirrors, closed-circuit
television, or an equivalent shall be provided to permit continuous observation
of the patient during irradiation and shall be so located that the operator can
observe the patient from the control panel.
ii. When the primary viewing system is by
electronic means, an alternate viewing system (which may be electronic) shall
be available for use in the event of failure of the primary viewing system.
4 Surveys, Measurements, Spot Checks and Operating Procedures.
a Surveys.
i. All CT x-ray systems installed after the
effective date of these regulations and those systems not previously surveyed
shall have a survey made by, or under the direction of, a qualified expert. In
addition, such surveys shall be done after any change in the facility or
equipment which might cause a significant increase in radiation
hazard.
ii. The registrant shall
obtain a written report of the survey from the qualified expert, and a copy of
the report shall be made available to the Agency upon request.
b Radiation Measurements.
i. The measurements of the radiation output
of the CT x-ray system shall be performed by, or under the direction of, a
qualified expert.
ii. The
measurement of the radiation output of the CT x-ray system shall be performed
annually and after any change or replacement of components which could cause a
change in the radiation output.
iii. The measurement of the radiation output
of the CT x-ray system shall be performed with a calibrated dosimetry system.
The calibration of such system shall be traceable to a national standard. The
dosimetry system shall have been calibrated within the preceding 2
years.
iv. CT dosimetry phantom(s)
shall be used in determining the radiation output of a CT x-ray system. Such
phantom(s) shall meet the following specifications and conditions of use:
i. CT dosimetry phantom(s) shall be right
circular cylinders of polymethyl-methacrylate of density 1.19 plus or minus
0.01 grams per cubic centimeter. The phantom(s) shall be at least 14
centimeters in length and shall have diameters of 32.0 centimeters for testing
CT x-ray systems designed to image any section of the body and 16.0 centimeters
for systems designed to image the head or for whole body scanners operated in
the head scanning mode.
ii. CT
dosimetry phantom(s) shall provide means for the placement of a dosimeter(s)
along the axis of rotation and along a line parallel to the axis of rotation
1.0 centimeter from the outer surface and within the phantom. Means for the
placement of dosimeters or alignment devices at other locations may be
provided.
iii. Any effects on the
doses measured to the removal of phantom material to accommodate dosimeters
shall be accounted for through appropriate corrections to the reported data or
included in the statement of maximum deviation for the values obtained using
the phantom.
iv. All dose
measurements shall be performed with the CT dosimetry phantom placed on the
patient couch or support device without additional attenuation materials
present.
v. The measurement of the radiation
output shall be required for each type of head, body, or whole-body scan
performed at the facility.
vi.
Measurement of the radiation output shall meet the following requirements:
i. The dose profile along the center axis of
the CT dosimetry phantom for the minimum, maximum, and midrange values of the
nominal tomographic section thickness used by the registrant shall be
measurable. Where less than three nominal tomographic thicknesses can be
selected, the dose profile determination shall be performed for each available
nominal tomographic section thickness.
ii. The CTDI
32 along the two axes specified in
1.6.9(4)(b)(iv)(ii) shall be measured. The CT dosimetry phantom shall be
oriented so that the measurement point 1.0 centimeter from the outer surface
and within the phantom is in the same angular position within the gantry as the
point of maximum surface CTDI identified. The CT conditions of
operation shall correspond to typical values used by the registrant.
iii. The spot checks specified in 1.6.9(4)(c)
shall be made.
vii.
Procedures for the measurement of radiation output shall be in writing. Records
of measurements performed shall be maintained for inspection by the
Agency.
c. Spot Checks.
i. The spot-check procedures shall be in
writing and shall have been developed by a qualified expert.
ii. The spot-check procedures shall
incorporate the use of a CT dosimetry phantom which has a capability of
providing an indication of contrast scale, noise, nominal tomographic section
thickness, the resolution capability of the system for low and high contrast
objects, and measuring the mean CTN for water or other
reference material.
iii. All spot
checks shall be included in the measurement required by 1.6.9(4)(b) and at time
intervals and under system conditions specified by a qualified
expert.
iv. Spot checks shall
include acquisition of images obtained with the CT dosimetry phantom(s) using
the same processing mode and CT conditions of operation as are used to perform
measurements required by 1.6.9(4)(b). The images shall be retained, until a new
measurement is performed, in two forms as follows:
i. photographic copies of the images obtained
from the image display device; and
ii. images stored in digital form on a
storage medium compatible with the CT x-ray system.
v. Written records of the spot checks
performed shall be maintained for inspection by the Agency.
d. Operating Procedures.
i. The CT x-ray system shall not be operated
except by an individual who has been specifically trained in its
operation.
ii. Information shall be
available at the control panel regarding the operation of the system and
measurements of radiation output. Such information shall include the following:
i. dates of the latest measurements and spot checks and the location within the
facility where the results of those tests may be obtained;
ii. instructions on the use of the CT
dosimetry phantom(s) including a schedule of spot checks appropriate for the
system, allowable variations for the indicated parameters, and the results of
at least the most recent spot checks conducted on the system;
iii. the distance in millimeters between the
tomographic plane and the reference plane if a reference plane is utilized;
and
iv. a current technique chart
available at the control panel which specifies for each routine examination the
CT conditions of operation and the number of scans per examination.
iii. If the measurement or spot check of the CT x-ray system identifies that a system operating parameter has exceeded a tolerance established by the qualified expert, use of the CT x-ray system on patients shall be limited to those permitted by established written instructions of the qualified expert.
31 The constant has a normal value of 1,000 when the Houndsfield scale of CTN is used.
32For the purpose of determining the CTDI, the manufacturer's statements as to the nominal tomographic section thickness for that particular system may be utilized.
Miss. Code Ann. § 45-14-11
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