C.8.15.3.8 CT Exposure Macro

C.8.15.3.8 CT Exposure Macro
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Table C.8-124 specifies the attributes of the CT Exposure Functional Group Macro.

Table C.8-124. CT Exposure Macro Attributes

Attribute Name	Tag	Type	Attribute Description
CT Exposure Sequence	(0018,9321)	1	Contains the attributes defining exposure information. Only a single Item shall be included in this Sequence.
>Exposure Time in ms	(0018,9328)	1C	Duration of exposure for this frame in milliseconds. If Acquisition Type (0018,9302) equals SPIRAL the duration of the exposure time for this frame shall be Revolution Time (0018,9305) divided by the Spiral Pitch Factor (0018,9311). See Section C.8.15.3.8.1. Required if Frame Type (0008,9007) Value 1 of this frame is ORIGINAL. May be present otherwise.
>X-Ray Tube Current in mA	(0018,9330)	1C	Nominal X-Ray tube current in milliamperes. Required if Frame Type (0008,9007) Value 1 of this frame is ORIGINAL. May be present otherwise.
>Exposure in mAs	(0018,9332)	1C	The exposure expressed in milliampere seconds, for example calculated from exposure time and X-Ray tube current. Required if Frame Type (0008,9007) Value 1 of this frame is ORIGINAL. May be present otherwise.
>Exposure Modulation Type	(0018,9323)	1C	A label describing the type of exposure modulation used for the purpose of limiting the dose. Defined Terms: NONE Required if Frame Type (0008,9007) Value 1 of this frame is ORIGINAL. May be present otherwise.
>Estimated Dose Saving	(0018,9324)	2C	A percent value of dose saving due to the use of Exposure Modulation Type (0018,9323). A negative percent value of dose savings reflects an increase of exposure. Required if Frame Type (0008,9007) Value 1 of this frame is ORIGINAL and Exposure Modulation Type (0018,9323) is not equal to NONE. Otherwise may be present if Frame Type (0008,9007) Value 1 of this frame is DERIVED and Exposure Modulation Type (0018,9323) is not equal to NONE.
>CTDIvol	(0018,9345)	2C	Computed Tomography Dose Index (CTDI_vol), in mGy according to IEC 60601-2-44, Ed.2.1 (Clause 29.1.103.4), The Volume CTDI_vol. It describes the average dose for this frame for the selected CT conditions of operation. Required if Frame Type (0008,9007) Value 1 of this frame is ORIGINAL. May be present otherwise.
>CTDI Phantom Type Code Sequence	(0018,9346)	3	The type of phantom used for CTDI measurement according to IEC 60601-2-44. Only a single Item is permitted in this Sequence.
>>Include Table 8.8-1 “Code Sequence Macro Attributes”			Defined CID 4052 “Phantom Devices”.
>Water Equivalent Diameter	(0018,1271)	3	The diameter, in mm, of a cylinder of water having the same X-Ray attenuation as the patient for this reconstructed slice (e.g., as described in [AAPM Report 220]).
>Water Equivalent Diameter Calculation Method Code Sequence	(0018,1272)	1C	The method of calculation of Water Equivalent Diameter (0018,1271). Required if Water Equivalent Diameter (0018,1271) is present. Only a single Item is permitted in this Sequence.
>>Include Table 8.8-1 “Code Sequence Macro Attributes”			Defined CID 10024 “Water Equivalent Diameter Method”.
>Image and Fluoroscopy Area Dose Product	(0018,115E)	3	X-Ray dose, measured in dGycmcm, to which the patient was exposed for the acquisition of the entire Irradiation Event from which this image and frame was reconstructed. Note AAll of the images and frames reconstructed from the same Irradiation Event will have the same value for this Attribute, which is the total for the Irradiation Event, which is repeated in each image or frame, regardless of whether or not the Irradiation Event UID (0008,3010) is present with a value in the Irradiation Event Identification Macro. I.e., the values for each image or frame should not be summed. The sum of the area dose products for each Irradiation Event may not result in the total area dose product to which the patient was exposed. This may be an estimated value based on assumptions about the patient’s body size and habitus. This value is required by [IEC 60601-2-63].

Note

The dose that a patient receives in a given procedure may be reported in one or more instances of the Radiation Dose Report SOP Class using TID 10011 “CT Radiation Dose”.

C.8.15.3.8.1 Exposure Time in ms Attribute (Informative)

An example of how the value of Exposure Time in ms (0018,9328) can be calculated:

The Exposure Time T_exposuretime of a volume shall be the time of the X-Ray radiation in position with this volume. It depends on the travel speed and the total collimation width.

T_exposuretime of the volume represented by a distinguished frame of a Spiral Scan can be calculated as follows:

T_exposuretime = N x S / V_travelspeed

CT_pitchfactor = DD / N x S

V_travelspeed = DD / T_{revolutiontime}

From this the exposure time can be derived:

T_exposuretime = N x S x T_{revolutiontime} / DD = T_{revolutiontime} / CT_pitchfactor

Legend:

DD = Table Feed per Rotation (0018,9310) in mm distance moved by the patient support in the z direction per 360° revolution in Spiral scanning

S = collimation of a single slice in mm

N = number slices

N x S = Total Collimation Width (0018,9307) in mm

V_travelspeed = Table Speed (0018,9309) in mm/second, travel speed of the patient table

T_{revolutiontime} = Revolution time (0018,9305) in seconds

CT_pitchfactor = Spiral Pitch Factor (0018,9311)

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