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Volume 9 Issue 3 - March 2011
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Flat Panel Volume CT

Flat panel volume CT (fpVCT) images cover larger volumes and have higher spatial resolution than multidetector CT

fpVCT allows visualization of fine details such as trabecular bone architecture

Each projection onto the flat panel detector is comparable to a standard plain film x-ray, and entire organs can be imaged in a single rotation

Continuous rotation allows depiction of dynamic processes such as blood flow

The radiation dose is comparable to that in multidetector CT imaging



Musculoskeletal Imaging
Vascular Imaging
Omni-scanning
Radiation Dose
Scheduling
Further Information
References

Flat panel volume CT (fpVCT) scanners have an innovative design that utilizes a 40 x 30 cm flat panel detector (Varian CB 4030) with an array of 2,048 x 1,536 detector elements. The detector panel is mounted in a rotating gantry and can produce images with a spatial resolution of 150μ x 150μ x 150μ. Multidetector CT scanners, whose detectors are mounted in rows, have a spatial resolution of approximately 0.4 mm in the x and y planes and 0.5 mm in the z plane. The projected image on a flat panel detector measures 18 cm in the z-axis, compared to 2-4 cm in 16- and 64- slice MDCT scanners. Therefore, volumetric data from an entire organ can be acquired with fpVCT in a single rotation of the gantry. With continuous gantry rotation, fpVCT can also be used for dynamic imaging of, -for example, blood flow in whole organs such as the brain or liver. fpVCT images also have reduced metal and beam hardening artifacts compared to MDCT.

However, fpVCT does have some limitations. The scintillator used in the flat panels (cesium iodide) is slower than that used in MDCT, which limits the temporal resolution of dynamic imaging. Contrast is also poorer, with the ability to discern differences of 5 Hounsfield units (HU) in fpVCT, compared to 3 HU or less in MDCT. Therefore, fpVCT will not replace MDCT but will offer advantages in some niche applications.

Figure 1. (A) Cross-sectional fpVCT image of a wrist bone and (B) 3D rendering of the same image..
Figure 1. (A) Cross-sectional fpVCT image of a wrist bone and (B) 3D rendering of the same image.


Musculoskeletal Imaging
The very high spatial resolution of fpVCT offers several advantages in musculoskeletal imaging. It is useful for visualization of the trabecular architecture of bone (Figure 1), imaging temporal bone anatomy, and cochlear implants (Figure 2).

Bone strength depends not only on the overall bone mineral density but also on trabecular structural anatomy, which has been shown to better distinguish between healthy and diseased bone than BMD. fpVCT has been used to demonstrate difference in apparent trabecular volume, number, and thickness, and separation in women with anorexia nervosa when compared with age-matched healthy controls. Finite element analysis, an engineering method that is used to evaluate stiffness and strength, has also been used to estimate stiffness and failure load in bone. With these methods, it is possible to document small changes in bone morphology that may occur after strength training and during recovery from, for example, spinal injury.

fpVCT shows much more detail than MDCT when imaging the temporal bones. For example, fpVCT can be used to evaluate middle ear pathology, the alignment of a stapes prosthesis, or the precise placement of a cochlear implant. In addition, fpVCT has been used to demonstrate bony invasion of chondrosarcoma or other tumors at the skull base.

Figure 2. Curved reformat images of cochlear implants from (A) MDCT imaging and (B), fpVCT imaging. Note the fpVCT images have superior spatial resolution and fewer image artifacts from the metal in the implant. Images courtesy of Soenke Bartling, DKFZ, Heidelberg, Germany.
Figure 2. Curved reformat images of cochlear implants from (A) MDCT imaging and (B), fpVCT imaging. Note the fpVCT images have superior spatial resolution and fewer image artifacts from the metal in the implant. Images courtesy of Soenke Bartling, DKFZ, Heidelberg, Germany.


Vascular Imaging
In vascular imaging, fpVCT has advantages not only because of its high spatial resolution and dynamic imaging capabilities but also because of its reduced metal and beam hardening artifacts. The high spatial resolution of fpVCT makes it better than MDCT for visualizing surface features of aneurysms, such as blebs and irregularities, as well as small adjacent vessels. Moreover, because metal streak artifacts are low, it is possible to use fpVCT instead of digital subtraction angiography to examine clipped aneurysms and to visualize, for example, neck remnants. Similarly, fpVCT has advantages for examining blood vessels with calcified atherosclerotic plaque or stents.

Dynamic fpVCT imaging can be used to image the flow of blood from the arterial phase to the late venous phase in continuous 4D imaging. This has potential uses in imaging brain perfusion, coronary blood flow, and visualization of arterial supply of tumors to assess for surgical resection. In addition, it has been shown that fpVCT can be used to image abnormalities in blood flow associated with subclavian steal phenomena, in which blood flow to the subclavian artery is delayed and arrives via retrograde flow in the vertebral artery.

fpVCT has also been found to be useful in assessing vessel patency of free tissue transfer in major reconstructive surgery. For example, a rectus abdominis myocutaneous free flap was transferred to salvage a devastating open leg fracture, which would otherwise result in amputation. The most common cause of failure of these grafts is a blood clot causing loss of venous outflow. Using fpVCT, it is possible to show that the graft is vascularized and that the blood flow arrives from the correct direction (Figure 3). If made more clinically available in the future, fpVCT could be used for pre-op evaluation of native vasculature, where vessels can be assessed for suitability as recipient conduits for free flap transfer. Currently, invasive angiograms are necessary to provide functional and anatomic data on vessels of a traumatized extremity.

Figure 3. (A) A traumatic myocutaneous defect in a 21 year old; (B) Rectus abdominis myocutaneous free flap reconstruction; (C) and (D). Two frames from a volume-rendered dynamic fpVCT image set showing the intact arterial and venous supply (arrows) in the TRAM flap. The transplant vessels and their surgical anastomoses can be visualized without obscuration from any metallic artifacts arising from the surgical clips and couplers. (Intra-operative images courtesy of Eric C. Liao, MD, PhD, Department of Plastic Surgery, Massachusetts General Hospital.)
Figure 3. (A) A traumatic myocutaneous defect in a 21 year old; (B) Rectus abdominis myocutaneous free flap reconstruction; (C) and (D). Two frames from a volume-rendered dynamic fpVCT image set showing the intact arterial and venous supply (arrows) in the TRAM flap. The transplant vessels and their surgical anastomoses can be visualized without obscuration from any metallic artifacts arising from the surgical clips and couplers. (Intra-operative images courtesy of Eric C. Liao, MD, PhD, Department of Plastic Surgery, Massachusetts General Hospital.)


Omni-scanning
In omni-scanning mode, an fpVCT scanner can be used alternately for fluoroscopy and tomographic imaging. In standard C-arm fluoroscopy, 2D projections are acquired at a rate of 25-30 frames per second. While acquisition of these high spatial resolution images is very fast, image contrast is limited. On the other hand, MDCT has high image contrast but relatively low spatial and temporal resolution. When an fpVCT scanner is used while the gantry is stationary, projection images can be acquired at a similar rate to a C-arm. These projection images can be complemented by obtaining tomographic images when the fpVCT gantry is rotated. Such combined imaging can be useful, for example, to determine the precise position of the tip of a catheter or for imaging the skull base during maxillofacial surgery.

Radiation Dose
Radiation dose is comparable to that from MDCT. However, it should be noted that if the spatial resolution is set at the highest level, the signal to noise ratio will be lower.

Scheduling
At this time, Mass General's single fpVCT scanner is not available for routine clinical use but is approved for clinical research, subject to IRB approval. The fpVCT is located in the Athinoula A. Martinos Center for Biomedical Imaging, Building 149, Charlestown Navy Yard. Please send email to Rajiv Gupta, MD, PhD , for scheduling information.

Further Information
For further questions on the flat panel volumetric CT, please contact , Mass General Imaging, at 617-726-7761.

We would like to thank Dr. Gupta, Ann Klibanski, MD, Chief, Neuroendocrine Unit, Massachusetts General Hospital, and Eric Liao, MD, PhD, Department of Plastic Surgery, Massachusetts General Hospital, for their advice and assistance in the preparation of this article.


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References

Gupta R (submitted). Temporal resolution of dynamic angiography using flat-panel volume CT: in vivo evaluation of time dependent vascular pathologies. Am J Neuroradiol.

Gupta R, Cheung AC, Bartling SH, Lisauskas J, et al. (2008). Flat-panel volume CT: fundamental principles, technology, and applications. Radiographics 28: 2009-2022.

Mitha AP, Reichardt B, Grasruck M, Macklin E, et al. (2009). Dynamic imaging of a model of intracranial saccular aneurysms using ultra-high-resolution flat-panel volumetric computed tomography. Laboratory investigation. J Neurosurg 111: 947-957.

Phan CM, Macklin EA, Bredella MA, Dadrich M, et al. (2010). Trabecular structure analysis using C-arm CT: comparison with MDCT and flat-panel volume CT. Skeletal Radiol.

Reichardt B, Sarwar A, Bartling SH, Cheung A, et al. (2008). Musculoskeletal applications of flat-panel volume CT. Skeletal Radiol 37: 1069-1076.

Walsh CJ, Phan CM, Misra M, Bredella MA, et al. (2010). Women with anorexia nervosa: finite element and trabecular structure analysis by using flat-panel volume CT. Radiology 257: 167-174.