British Journal of Radiology 74 (2001),323-327 © 2001 The British Institute of Radiology
Development of a protocol for coronal reconstruction of the maxillofacial region from axial helical CT data
E G Hoeffner, MD
1
D J Quint, MD
1
B Peterson, BA
1
E Rosenthal, MD
2 and
M Goodsitt, PhD
1
1 Department of Radiology, Division of Neuroradiology
2 Department of Otolaryngology—Head and Neck Surgery, University of Michigan Medical Center, 1500 East Medical Center Drive. Ann Arbor, MI 48109, USA
Correspondence: Douglas Quint, MD, B1D520, Division of Neuroradiology, Box 30, Department of Radiology, University of Michigan Medical Center, 1500 East Medical Center Drive, Ann Arbor, MI 48109, USA
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Abstract
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Using a fresh frozen cadaver head, a series of axial helical CT scans were obtained using varying imaging parameters both before and after traumatizing the head. The appearance of reformatted coronal images was optimized for the lowest radiation dose. A protocol for imaging the maxillofacial region was developed that produced diagnostic coronal reconstructed images from the axial helical CT data.
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Introduction
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Helical (spiral) CT acquires data in a continuous fashion, allowing the use of image reconstruction algorithms to produce multiplanar reformatted images. Recent advances in CT technology now permit rapid acquisition of thin section axial CT data in a reasonable amount of time. Our CT scanner ("Lightspeed"; General Electric Medical systems, Milwaukee, WI) is capable of obtaining 1.25 mm sections through the maxillofacial region in less then 20 s without delays for tube cooling. Owing to its recent introduction, there are currently no protocols for performing head and neck imaging using such a scanner. Although it is known that thin slice (1 mm) axial helical CT data can be reformatted to produce good quality images, there is limited routine clinical utilization of reformatted images, in part because thinner section helical CT imaging on conventional scanners requires long scanning and tube cooling times (up to 17 min at our institution) [1–5]. Moreover, studies have shown that coronal images reformatted from axial helical data acquired under standard conditions (3 mm collimation with a 1:1 pitch) are inadequate for the assessment of facial fractures oriented primarily in the axial plane [6].
There are several clinical settings where direct coronal maxillofacial imaging may be desired but cannot be obtained in a timely fashion, for instance in patients with facial trauma who also have suspected cervical spine injuries, and in intubated intensive care patients with suspected paranasal sinus inflammatory disease. Multiply injured trauma patients are often brought to the radiology suite for CT evaluation of the head, chest, abdomen and pelvis prior to plain radiographic assessment of the cervical spine. At that time, if there is a high suspicion for mid-facial or orbital fractures, it is common practice to also obtain axial maxillofacial images for diagnosis and pre-operative planning while the patient is already in the scanner. However, adequate assessment of the cribriform plate, orbital roof, orbital floor and planum sphenoidale, structures that are primarily oriented in the axial plane, requires imaging evaluation in the coronal plane. Direct coronal imaging can only be obtained once the cervical spine has been "cleared" in these patients, which may take up to 24–48 h after arrival in the Emergency Department at our institution. Moreover, multiply injured trauma patients may remain intubated for a week or longer, further delaying complete (i.e. direct coronal) maxillofacial and orbital evaluation.
Return visits to the CT suite for direct coronal imaging results in significant inconvenience and cost to the patient, is often performed too late to be clinically helpful, and subjects the patient to additional ionizing radiation in particularly sensitive areas such as the ocular lens. A substantial improvement in patient care could be made if radiologists and clinicians were able to routinely utilize coronal images reformatted from axial images obtained at the time of patient's initial evaluation. This study provides a protocol for the routine use of reformatted coronal images in maxillofacial trauma patients using a late generation helical CT scanner.
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Materials and methods
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A fresh cadaver head was obtained from the Anatomical Donations Office at our institution and underwent imaging using a late generation multislice helical CT scanner ("Lightspeed"; General Electric Medical Systems, Milwaukee, WI). Imaging of the maxillofacial structures was performed in the axial plane with the following constant parameters: tube potential of 120 kVp; detector configuration 4 mmx1.25 mm; HQ scan mode with a pitch of 3:1; table speed 3.75 mm per rotation; displayed field of view (DFOV) 18.0 cm. The helical algorithm was used to reconstruct ("retroreformat") the data into axial images with a 1.25 mm slice thickness, which is presently the minimum possible on this scanner, at 0.6 mm intervals. The milliamperage, which directly correlates with the radiation dose delivered to tissue, was varied from 100–400 mAs and the machine-calculated radiation dose was recorded. Next, scan data were transferred to a GE Advantage Windows workstation (General Electric Medical Systems, Milwaukee, WI) where the helical CT data underwent multiplanar reconstruction (in the coronal plane) with a slice thickness of0.4 mm, which is the thinnest possible. Reformatted slice intervals were varied between 0.4 mm and 5 mm.
The cadaver specimen was also scanned using our standard direct axial and coronal imaging protocol for this CT scanner. The parameters used for axial images were: 120 kVp; 290 mAs; detector configuration 4 mmx1.25 mm; HQ scan mode with a pitch of 3:1; table speed 3.75 mm per rotation; DFOV 18.0 cm; slice thickness 2.5 mm. The parameters used for direct coronal images were: 120 kVp; 290 mAs; detector configuration 4 mmx2.5 mm; 4i scan mode; DFOV 18.0 cm; slice thickness 2.5 mm.
Films were printed and reviewed independently by two staff neuroradiologists to evaluate subjectively the axial and reformatted coronal images for bone and soft tissue quality in comparison with the standard axial and direct coronal images. Optimal scanning parameters at an acceptable radiation dose were determined.
The cadaver specimen then underwent surgical manipulation by an otolaryngologist, resulting in fracture of structures primarily oriented in the axial plane, to determine whether the selected scanning parameters could detect these fractures. Specifically, a minimally displaced fracture of the orbital floor was created using a transconjunctival approach. The cribriform plate and the orbital roof were also fractured using an intracranial approach. All fractures were confirmed by direct visualization to be non-displaced or minimally displaced (<1 mm). The cadaver head underwent repeat imaging within a narrow range of variables as established by imaging of the pre-trauma specimen (120 kVp; 120 mAs; detector configuration 4 mmx1.25 mm; HQ scan mode with a pitch of 3:1; 3.75 mm per rotation table speed; 18.0 cm DFOV; 1.25 mm axial slice thickness retroreformatted at 0.6 mm intervals). Coronal reconstructed images were obtained with a varying slice interval as described above. Repeat standard axial and direct coronal images were also obtained on the post-surgical cadaver head. All scanning took place during three 1–2 h sessions over a 15-day period. Two neuroradiologists then independently evaluated bone and soft tissue images obtained from the fractured cadaver head. The quality of axial and reconstructed coronal images relative to direct coronal imaging was determined with respect to the ability to detect the fractures oriented primarily in the axial plane.
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Results
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The fresh frozen cadaver head underwent CT imaging using the helical CT scanner with varying milliamperage, listed in Table 1
with the corresponding radiation exposure. Review of the axial and reconstructed coronal images obtained with a milliamperage between 100 mA and 400 mA showed that a dose between 100 mA and 200 mA was adequate for visualization of pertinent soft tissue structures, namely the optic nerve and the extraocular muscles. No definite improvement in image quality was appreciated at doses above 200 mA. Imaging was then repeated at 120 mA, 150 mA and 170 mA. These scans did not show a substantial subjective improvement in image quality above 120 mA, but did reveal a noticeable decrease in image quality compared with 100 mA (Figure 1). Specifically, at 100 mA the soft tissue and bone margins were observed to be less sharp than at 120 mA. Dosimetry information was obtained from a machine-computed CT dose index (CTDIw) that gives an accurate measurement with respect to calculated radiation doses at our institution. The machine-calculated radiation dose was acceptable at 120 mA (6.7 cGy), particularly in comparison with the standard axial and coronal maxillofacial CT, which had a radiation dose of 17.3 cGy. Using the Advantage Windows software, multiplanar reconstructions (MPVR model building) produced optimal quality reformatted images with a slice thickness of 0.4 mm and a slice interval of 1.0 mm, slice intervals 
1.5 mm resulting in poorer visualization of fractures. These parameters resulted in reformatted images that demonstrated the fractures that had been created in the cadaver head by surgical manipulation (Figures 2 and 3). In addition, the reformatted coronal images were not degraded by streak artefact from dental fillings as were the direct coronal images (Figure 3).
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Table 1. Milliamperage (mA) vs machine-calculated radiation dose (axial helical scanning through the maxillofacial region)
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Figure 1. Reformatted coronal CT images with varying mAs. (a) Image obtained with a milliamperage of 100 mA shows blurred bone margins. (b) At 120 mA the bone margins appear sharper. Apparent differences in the amount of intracranial and maxillofacial gas and soft tissue on this and subsequent images are related to the manipulation of and change in position of the cadaver head that was necessary for direct axial and coronal scanning.
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Figure 2. (a) Direct coronal CT image with a slice thickness of 2.5 mm demonstrates a minimally displaced left cribriform plate fracture (open arrow). (b)0.4 mm reformatted coronal image with a slice interval of 0.4 mm demonstrates the discontinuity and depression of the cribriform plate (arrow). This could be seen on four contiguous reformatted images. (c)0.4 mm reformatted coronal image with a slice interval of 1.0 mm also demonstrates the fracture (arrowhead). The fracture was adequately demonstrated with these parameters and was seen on two contiguous reformatted images.
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Figure 3. (a) A right orbital roof fracture is clearly seen on the direct coronal image with a slice thickness of 2.5 mm (open arrow). Streak artefact from dental fillings degrades the image. (b) 0.4 mm reformatted image with a slice interval of 1.0 mm adequately demonstrates the fracture (solid arrow) and the streak artefact is eliminated.
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Discussion
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Coronal imaging is required for adequate evaluation of patients with maxillofacial trauma. Although recent advances in CT technology now permit the acquisition of thin (1.0–1.25 mm) axial CT sections in a reasonable amount of time, which can then be used to generate good quality coronal reformatted images, to our knowledge no protocols have been developed to implement such scanning on a routine clinical basis in patients with maxillofacial trauma. It has recently been demonstrated that thin section (0.5 mm and 1.0 mm) axial CT scanning can be used to generate high quality reformatted coronal images for evaluation of the petrosal bone and the paranasal sinuses, respectively [4, 5]. This study was designed to develop a protocol for obtaining diagnostic coronal reformatted images from axial helical CT data while ensuring acceptable radiation doses to patients.
A fresh frozen cadaver head was used to ensure that the tissue attenuation closely approximated that of living tissue. The minimum axial CT slice thickness (1.25 mm) allowable on our late generation multibeam helical scanner was chosen for axial image reconstruction as it has been shown that thin axial slices are necessary to obtain diagnostic coronal reformatted images. A pitch of 3:1 was used which is the minimum pitch possible on our scanner. Klevansky [5] reported varying degrees of stair-step artefact on coronal reformatted images depending on the pitch; however, this artefact was not a significant problem in our protocol. Inspection of images obtained at varying mAs demonstrated adequate image quality with 120 mA. At 100 mA, the image quality was significantly degraded and there did not appear to be a subjective improvement in image quality at 150 mAs or greater. The radiation exposure to the patient at 120 mA was also within currently acceptable limits and was less than that for our standard axial and coronal imaging protocol for the "Lightspeed" scanner, where the higher radiation dose is largely a factor of the higher mA. Since this study was completed, a focal-spot tracking system has been introduced on this scanner that decreases radiation dose from 6.7 cGy to 3.8 cGy for the reformatted protocol and from 17.3 cGy to 10.3 cGy for the standard protocol [7].
The minimum possible slice thickness (0.4 mm) was selected for the coronal reformatted images generated on the GE Windows Workstation to produce images with the best spatial resolution. The interval between the reformatted coronal slices was varied from 0.4 mm to 5.0 mm. A slice interval of 1.0 mm produced images of diagnostic quality that enabled detection of the fractures produced in the cadaver head. A slice interval 1.5 mm resulted in poor delineation of some fractures. A thinner slice (0.4 mm) interval did not result in significant improvement in fracture detection, but generated an unwieldy number of images (approximately 250 images). Even with the parameters that we decided upon for this protocol, approximately 100 reformatted images were generated.
The protocol described in this study (Table 2) produced reformatted coronal images that appear to be of diagnostic quality and may be able to replace direct coronal imaging in some patients. While this protocol was developed using experienced imagers, it was not a double-blinded study. However, we recently completed a study of blunt trauma in cadaver heads that appears to confirm the validity of this protocol [8]. Finally, we recognize that the parameters used in this study may not be optimal for scans performed on multidetector scanners from other manufacturers owing to differences in detector dose efficiency, tube potential, beam filtration, scan geometry and slice thickness.
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Footnotes
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Funded in part by the General Clinical Research Center (GCRC) as per Grant No. M01 RR00042 from the National Center for Research Resources, National Institutes of Health. 
Received for publication May 30, 2000.
Revision received November 14, 2000.
Accepted for publication December 5, 2000.
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References
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Abstract
Introduction
