Reduction in Exposure Rates during Realtime Computerized Axial Tomography Interventional Procedures

1
Reduction in Exposure Rates during Real-time
Computerized Axial Tomography Interventional
Procedures

• Dave Tripp, Ph.D.
• Karen Langley, M.S.
• William Orrison, M.D.
• Department of Radiology
• University of Utah Health Sciences Center
• Salt Lake City, UT
• Radiological Health Department
• University of Utah
• Salt Lake City, UT

2
Introduction

• CT fluoroscopy or fluoro CT provides real-time
  reconstruction and display of CT images.
• This modality has advantages over conventional CT
  in performing certain types of interventional
  procedures such as chest biopsies.
• Primarily by reducing the time required to
  perform complex interventional procedures.
• However, fluoro CT can produce large increases in
  primary radiation dose to patients and secondary
  radiation (scatter) to attending staff by a
  factor of 10-20 times that of conventional
  fluoroscopy.

3
Fluoro - CT Entrance and Scatter Exposure Rate
Measurements
4
Possible Remedies

• By reducing imaging times
• By reducing x-ray technique factors such as kVp
  and mA.
• By using a portable lead shield adjacent to the
  imaging plane to shield attending staff from
  scattered radiation.

5
Response

• Imaging time is a function of difficulty of
  procedure, anatomy of patient, and radiologist
  technique. In our Department, imaging time has
  been cut to less than 3 min.
• The fluoroscopic technique factors cannot be
  lowered below certain values in order to maintain
  minimum image quality. At the lowest technique
  used at our institution, the dose rates is still
  10 times conventional fluoroscopy.
• The use of a portable lead shield creates
  problems with maintaining sterile conditions, and
  blocking physicians access to the biopsy site.

6
Alternate Solution?

• Place a sterile drape over the biopsy site.
• Use a surgical drape that is composed of a
  relatively high Z material -Bismuth in our case
  (which is easily disposed of)- to shield
  attending staff from scattered radiation and
  partially shield the patient from primary
  radiation.
• Cut a small opening in the surgical drape for the
  purpose of biopsy needle placement.

7
Bismuth Shield on Anthropomorphic Phantom with
Biopsy Needle in Place
8
Purpose of the Investigation

• With the use of an anthropomorphic chest phantom
  determine the patient entrance dose rates, and
  attending staff scatter dose rates.
• To use the bismuth surgical drapes with various
  lead equivalencies to determine their
  effectiveness in
• reducing patient entrance exposure
• reducing exposure to attending staff to scatter,
  and
• in determining the degree of associated image
  degradation.

9
Experimental Methods

• Patient entrance dose was determined using TLDs
  on top of and underneath the drapes.
• Attending staff exposure to scatter was measured
  using an ion chamber placed at the approximate
  position of the physicians face.
• 18-gauge biopsy needle placed in phantom lung
  field was imaged to evaluate the degree of
  distortion.

10
Results

• Patient entrance dose
• skin entrance dose decreased 65-75 in the region
  covered by the drape using the lowest CT
  technique with the optimal surgical drape.
• Obviously, the remainder of the body not covered
  by drape would not receive the the benefit of the
  skin entrance dose reduction.

11
Results (continued)

• Scattered radiation to attending staff
• scatter rate also significantly reduced
• scatter rate not significantly reduced beyond 0.2
  mm lead equivalency.
• Scatter from the phantom other than that
  attenuated by drape seemed to dominate beyond 0.2
  mm lead equivalency.

12
CT Fluoro Scatter Rate and Reduction as a
Function of Shielding (lead equivalency)
13
Results (continued)

• Image degradation
• bismuth drapes with up to 0.4 mm lead equivalency
  produced acceptable images
• image degradation in area of needle puncture site
  was minimal
• image of needle in the region of interest (biopsy
  site) was of sufficient quality so as not to
  impede the required precision in placing the
  needle in the lesion.

14
Axial CT Image of Chest Phantom with Simulated
Needle Biopsy - No Shielding
15
With 0.273 mm Lead Equivalency Shielding
16
With 0.359 mm Lead Equivalency Shielding
17
With 0.430 mm Lead Equivalency Shielding
18
Discussion

• The advent of CT fluoro has provided a more rapid
  and easy means of performing interventional
  procedures when a detailed and real-time 3
  dimensional image is of benefit.
• Even when used under optimum conditions, the
  exposure rates to patient and staff are
  significantly greater than either conventional CT
  or most routine fluoroscopic procedures.
• The use of sterile surgical drapes loaded with
  bismuth is a simple way to reduce staff exposure
  by as much as 50 without significant degradation
  of the patient image.

19
Discussion (continued)

• The problem associated with placing a shield on
  the patient to prevent scatter during
  conventional fluoroscopy is the significant
  degradation of the image as well as an increase
  in skin entrance dose caused by the auto
  brightness control associated with a fluoroscopy
  unit.
• However, with CT fluoro the image is not acquired
  using x-rays emanating from a single direction.
• In fact, the CT fluoro image is not reconstructed
  for each rotation, but rather every 60o . Thus
  the remaining views make it possible to obtain
  adequate image quality in the area of interest.

20
Conclusion

• The use of CT fluoro has definite benefits that
  may include significant savings in procedural
  time, ease of access, and in some cases, even
  procedural viability.
• The trade-off is the potential for excessive
  exposure of patient and staff to increased
  amounts of radiation.
• Our investigation shows that with the use of
  shielding materials, such as bismuth -containing
  sterile surgical drapes can lower dose to the
  patient, and provides significantly less
  occupational exposure to attending staff while
  maintaining a sterile field and allowing the
  radiologist easy access to the biopsy site.