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Robust CTA in a non-IQon world

Abstract
Philips CT Clinical Science Philips Healthcare • USA

Dr. med. Valentin Fretz

Head of CT Division, Vice Director

Institute for Radiology and Nuclear Medicine

Kantonsspital Winterthur

Brauerstrasse 15

8400 Winterthur

Switzerland

valentin.fretz@ksw.ch

 

Background: More challenges in CT angiography (CTA)

Advances in vascular surgery and interventional radiology and worldwide adoption of new treatment procedures in the last decade shifted the role of CTA from a former sole diagnostic more and more into an additional treatment planning tool. Today CTA allows computer-aided 3-dimensional exact quantification of vascular stenosis and aneurysms, assessment of the local anatomy and in the same scan evaluation of the potential therapeutic access. Therefore CTA is an ideal tool for patient specific 3-dimensional treatment planning, for example TAVI- and EVAR-Planning. With the newer techniques in plastic surgery also the preoperative assessment of the very small arteries by CTA especially at the donor site in different flap reconstruction procedures is more and more important.

 

Therefore optimal image quality in CTA of the greater and also the very small arteries is essential for proper treatment planning in various procedures (with or without computer-aided 3-dimensional post-processing).

 

In the following clinical examples of two somewhat challenging examination types simple rules for scan planning and image reconstruction with IMR are proposed to guarantee robust CTA acquisition and image quality.

 

Simple rules for optimal scan planning

To achieve an optimal image quality in CTA on the one hand, different patient specific properties have to be taken into account already at the time of scan planning as for example general and local circulation as well as local anatomy. This is a classic situation for so called patient centered imaging (PCI). On the other hand simple and easy applicable rules for scan planning are very helpful in daily routine. As fortunately human physiology has not changed over time, these rules can be derived from the results of many earlier studies on physiologic and pathophysiologic conditions relevant for CTA planning. Many relevant factors have been studied and are therefore predictable; individual differences in cardiac output and contrast media transit time (CMT) are handled with bolus tracking techniques (Ref. 1-3). The desired arterial attenuation profile in the region of interest can be achieved best with optimal contrast media injection parameters. The acquisition can be best timed within the expected arterial attenuation profile with optimal individual scan parameters, especially scan time and post threshold delay in bolus tracking techniques.

 

In the following clinical examples two simple different examination types are proposed for typical clinical conditions: Fixed long injection time and long scan time in the CTA of the upper extremities and biphasic injection with short scan time of the very small arteries.

 

Improvement with newer CT Techniques: IQon and IMR

IQon Spectral CT provides many new very helpful options also in CTA. As in many other sites around the world IQon is not available at our department at the moment. To our experience IMR (Iterative Model Reconstruction; Ref. 4) is very helpful and ideal feature to improve thin slice CTA image quality in comparison to iDose4, especially in low voltage acquisition with high concentrated contrast media (Figure 1A+B). Several “older” CT scanner families from Philips can easily be upgraded with IMR to improve image quality without the need to provide a complete new CT Scanner for that aspect.

 

Robust CTA of the upper extremity

Patient preparation includes a prone Superman position of the patient if possible with a contralateral cubital iv access. Similar to run-off CTA of the lower extremity also in the upper extremity a fixed long injection time and scan time is a robust technique to prevent outrunning the contrast bolus in a single helical acquisition from the aortic arch to the fingertips. To achieve an appropriate distribution of the radiation dose, 3D modulation (mAs) is highly recommended. Finally IMR reconstruction technique is very helpful to guarantee the desired image quality in thin slice CTA reconstruction also in 100 kV scans including the otherwise problematic region of the shoulders. If both arms have to be evaluated, either a single scan with central venous access for contrast media administration or two separate examinations each with a contralateral cubital iv access can be done (Ref. 5). Examples: Figure 2 + 3.

 

Robust preoperative CTA evaluation of common perforator arteries in plastic surgery

CTA is the most accurate method for the preoperative assessment of perforator arteries at the donor site in many autologous flap reconstruction procedures in plastic surgery leading to a significant reduction of flap failure rate, other complications, duration of the operation time and inpatient stay at hospital (Ref. 6-8). Free deep inferior epigastric artery perforator (DIEP; Figure 1) and superior gluteal artery perforator (SGAP; Figure 4) flaps are typically used for breast reconstruction. Thoracodorsal artery perforator flap (TDAP) and lumbar artery perforator flap (LAP; Figure 5) are used to reconstruct or cover more superficial defects. Preoperative assessment includes the evaluation of the perforator quality and type as well as the localisation of the best perforators in relation to intraoperative helpful landmarks such as for example umbilicus (DIEP), posterior superior iliac crest and trochanter major (SGAP), inferior edge of the scapula (TDAP) or iliac crest (LAP).

 

Ideal patient positioning depends on the desired perforator type and operation technique. To achieve a maximal enhancement of the small perforator arteries without venous contrast overlay we recommend a biphasic injection protocol with high iodine flux/injection rate at the beginning and low flowrate afterwards to benefit from contrast media recirculation during the prolonged injection time. As CMT is handled with bolus tracking a fast scan acquisition is best timed at the end of injection time (with a correspondingly long post threshold delay) at the moment of the estimated maximal enhancement.

 

Figure 1A
Figure 1A

CTA of left umbilical DIEA-Perforator (same examination); same position, axial slice thickness (0.9 mm) and average window (WL 10/WW 400) each. On the left, reconstruction with IMR Level 1 and Soft Tissue setting and on the right reconstruction with iDose4 Level 3 and B-Filter.

 

Figure 1B
Figure 1B

CTA of left umbilical DIEA-Perforator (same examination); same sagittal position, slice thickness (10 mm) and MIP window (WL 20/WW 350) each. On the left, reconstruction with IMR Level 1 and Soft Tissue setting, on the right reconstruction with iDose4 Level 3 and B-Filter.

 

Figure 2
Figure 2

CTA of the left arm: 3D volume rendering and curved MIP in a patient with false aneurysm and hematoma after puncture of an AV-shunt for dialysis.

 

Figure 3
Figure 3

CTA of the left arm: 3D volume rendering and MIP in a patient with symptomatic perfusion deficite in the left hand after coronary intervention. Stenosis of the subclavian artery and local dissection of the axillary artery. No lesions at the peripheral arteries.

 

Figure 4
Figure 4

CTA of the left superior gluteal artery perforators (circles) before breast reconstruction. 3D volume rendering and axial MIP.

 

Figure 5
Figure 5

CTA of the third lumbar artery perforator above the left iliac crest at the lumbar trigonum in 3D volume rendering an axial MIP (with measurements).

 

References

  1. Fleischmann et al; Europ Radiol 2002; 12:S11-16.
  2. Fleischmann et al. How to design injection protocols for multiple detector-row CT angiography (MDCTA). Europ Radiology 2005, 236:1076-82.
  3. Fleischmann et al. Radiology 2005, 236:1076-82.
  4. Thomas Morton: Basic IMR testing, considerations and image quality trends. Philips Iterative Model Reconstruction (IMR) white paper.
  5. Bozlar et al. CT angiography of the upper extremity arterial system. AJR 2013; 201:745–752.
  6. Scott et al. Computed tomographic angiography in planning abdomen-based microsurgical breast reconstruction: A comparison with color duplex ultrasound. Plast Reconstr Surg 2010;125(2):446–453.
  7. Malhotra et al CT-guided deep inferior epigastric perforator (DIEP) flap localization - Better for the patient, the surgeon, and the hospital. Clinical Radiology 2/2013; 131-138.
  8. O’Connor et al. Preoperative computed tomography angiography for planning DIEP flap breast reconstruction reduces operative time and overall complications Gland Surg. 2016 Apr; 5(2): 93–98.


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Abstract
13th MDCT Users Meeting abstracts, 3D, aneurysm, Body, bolus tracking, iDose4, image quality, IMR, Interventional, iterative model reconstruction, peripheral runoff CTA, stenosis, TAVI, upper extremity, Vascular
 

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