Magnetic resonance of adolescent idiopathic scoliosis

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1 Australian Institute of Radiography The Radiographer 2005; 52 (2): 7 14 Magnetic resonance of adolescent idiopathic scoliosis University of Queensland, St Lucia Campus, Brisbane, Queensland, Australia Correspondence to peterlavery@hotmail.com Abstract The following work aims to assess the feasibility of the magnetic resonance imaging (MRI) technique in the assessment of adolescent idiopathic scoliosis (AIS).At present, three modalities have proven to be worthy in this application. These are general radiography, computerised tomography (CT) and MRI. General radiography and CT have been the traditional methods, but carry some significant disadvantages including the use of ionising radiation. MRI, given its reliability, accuracy and lack of ionising radiation, has become the modality of choice for a wide range of applications including neurological and spinal conditions. In this study, measurements of axial images of the scoliotic spine were performed and inter-observer variations were compared. The study, using MRI techniques, demonstrated favourable and promising results. In four of the measurements performed, absolutely no variations were found. The study also demonstrated a high positive correlation between curve data obtained from general radiography, and the curve data from those same patients using 3D coronal MRI imaging. Introduction Scoliosis is a deformity which causes the lateral curvature of the spine along with vertebral rotation. The spine is normally straight in the frontal plane and if it is not, then there is a scoliosis (right/ left deformity). 1 Idiopathic scoliosis is a structural curve with no clear underlying cause. There are three forms of scoliosis. These are Infantile, Juvenile and Adolescent, with the latter accounting for the majority of cases of idiopathic scoliosis. 2 Due to its prevalence, this study will focus on the evaluation of patients with adolescent idiopathic scoliosis (AIS). Currently, three imaging techniques are utilised for the assessment of AIS. They are general radiography, computerised tomography (CT) and more recently, magnetic resonance imaging (MRI). General radiography has several flaws. First, the direction of the X-ray beam is fixed and cannot be controlled to image angled vertebrae whose tilt is unknown. Second, the degree of deformity in the axial plane can be difficult to determine. Third, inherent inaccuracies can result due to loss of definition of anatomic landmarks such as the vertebral endplates and pedicles. Over the last decade, scanning and analysing the spinal column in three dimensions has become possible using CT and MRI techniques. The ability to assess the degree of rotation in all three planes is not only a helpful prognostic factor, but can assist greatly in the surgical planning for correction of the scoliotic curve. At present, CT is used very commonly in the assessment of AIS pre-surgically. This is mainly due to the high availability of the modality, the cost of the examination, and the time efficiency of performing a scan. Multi-slice CT has proven its worth in providing accurate imaging data on the scoliotic spine with good spatial resolution. Images today are acquired at a thickness of 1 mm and an interval of mm. The disadvantage of this technique is the ionising radiation dosage. Typical skin doses range from milli Gray (mgy) during head scans and mgy during body scans. 4 These values are only approximate and vary depending on the type of CT scanner and the technique of the examination. This factor also becomes highly important when considering repeated examinations and follow-up studies in adolescent and juvenile patients. MRI can provide an alternative. MRI is a very capable imaging modality. It uses a magnetic field, typically Tesla, radio frequencies and a gradient system to produce an image. In MRI, exquisite spatial and contrast resolution make it possible to assess spinal cord and canal, the spinal column and the muscles surrounding the spine in great detail. Perie et al. 5 have shown MRI to be a favourable technique in imaging intervertebral disc morphology including nucleus migration. In evaluating the feasibility of using MRI of AIS patients in the clinical setting, it is important to identify the three main safety considerations in the practice of MRI. These are exposure to: Radio frequency (B1) In MRI, tissue warming is a primary concern. Specific absorption rate (SAR) is the key indicator on the effects of RF fields and is measured in Watts/kg. SAR thresholds have been assigned for different anatomical reference regions and should not exceed 8 w/kg over the body trunk. 6 The static magnetic field (Bo) Ferromagnetic objects brought close to the magnetic field will experience an attractive force, which can cause potential risk to the patient, operators and equipment. Time varying magnetic fields (gradient systems) Gradient fields are magnetic fields that rapidly change with time. They are measured as the rate of change of the magnetic field over time. The Food and Drug Administration (FDA) of the USA 7 has thresholds in place to limit the switching rates of the field for each part of the body. The gradient system also produces strong acoustic noise. This noise is in the range of db. 8 The FDA has advised that the maximum allowable acoustic noise level is 140 db. 8 Other factors that require consideration include cost to the patient, time of examination, MRI and the pregnant patient, and artefacts such as susceptibility to metal, aliasing, and RF inhomogeneity. Although it has not been proven conclusively that MRI has a detrimental effect on the pregnant patient, the

2 8 The Radiographer examination is usually only performed if it cannot wait until full term and it is clinically neccessary to be performed urgently. The accuracy of MRI in evaluating axial rotation of affected vertebral bodies has been excellent, with past documented calculations having statistically insignificant interobserver variations. 9 The MRI procedure is currently used almost exclusively for three main indications. These are: unusual pain or an abnormal neurological examination, the presence of a left thoracic curve and the evaluation of tumours, syringomyelia, or spondylolithesis. In 2001, a study was performed that determined that only 2% in a study of 327 AIS patients (with normal neurological examinations) had any pathology related to the above indications. 2 If proven suitable, perhaps the application of the MRI technique could be stretched further (from the bounds of the above indications) to become more main stream for the pre-surgical assessment of AIS. Aim MRI technology has had a significant impact in the understanding of the scoliotic deformity. 3 Images produced in MRI demonstrate exquisite soft tissue delineation, with excellent spatial resolution and high accuracy in displaying the spine and its surrounding structures. The aim of this project is to use current literature as a base and some clinical scanning to emphasise the benefits of the MRI technique in patients with scoliosis, and prove its accuracy in the assessment of the scoliotic curve. Hypothesis This project will prove that MRI is a worthy and capable technique in evaluating AIS, by investigating the benefits of MRI in respect to other imaging alternatives, evaluating the accuracy of the MRI technique on the scoliotic spine and providing a robust protocol for the assessment of the scoliotic curve. Method Recruitment of subjects Subjects for the project were a selection of five scoliosis patients from The Mater Hospital Spinal Clinic in Brisbane. The patients ranged in age from 14 to 38 years. Written support and consent was received from The Mater Hospital Spinal Clinic to perform this clinical study. Suitable patients were identified as those who had been diagnosed with AIS and had met all MRI safety criteria. Consent was also obtained from each individual and appropriate screening procedures for contraindications followed. For example, female patients were given a pregnancy test; this was achieved using a home pregnancy tests (HPT) for the identification of the hcg hormone. Data collection After ethical clearance was reached from both The Wesley Hospital and University of Queensland ethics boards, a period of approximately two weeks was utilised to obtain data from the MRI units at The Wesley Hospital. The patients were imaged using modified spine protocols designed to assess the spinal curvature. These protocols are discussed later. The images were produced using a Siemens Sonata 1.5T magnetic resonance scanner along with a phased array spine coil. Data was acquired in three imaging planes of the spine: primarily sagittal and axial. The sagittal acquisitions used section thicknesses of 1.1 mm at a 0 interval. Certain endplates 3 (at the apices of the scoliotic curve) on each individual were selected and axial slices performed through them. Image sequences and parameters Sequence A Axial T2 block acquisitions of five slices each were performed Figure 1 (a) Custom protocol Figure 1(b) CT demonstrating measurement protocol from Vaccaro et al. 10 at three specific levels. These levels were chosen based on the top, middle and bottom of the scoliosis curvature. Each group of slices was centred to the mid-body of the selected vertebrae and parallel to the endplates of the vertebral body. Axial images could then fully demonstrate the required anatomical references for geometrical measurement and analysis. It should also be noted that spinal surgeons use the mid-body level for their assessment of vertebral rotation prior to surgical intervention The parameters for the axial acquisition were: Parameters Turbo spin echo T2 axial Repetition time (TR): 4000 Echo time (TE): 98 Slices: 5 Distance factor: 10 Matrix: 271 read x 512 phase Slice thickness: 2 mm Number of signals averaged (NSA): 3 Scan time: 3.54 min (for each level) Sequence B The final sequence was a three dimensional isotropic data set acquired in the sagittal plane. Planned from the coronal localiser, a number of slabs from 6 10 were selected to cover the extent of the curve from the left extreme to the right extreme. The block was acquired using a 50% slice over sampling rate, giving increased signal and better overall image quality. 3 Because data was acquired in a block fashion, imaging could be manipulated in all three planes to form excellent representations of the spine with incredible resolution and accuracy. Parameters TR: 3000 Slab: sagittal orientation NSA: 2 TE: 110 Scan time: 9.41 min Slice Over sampling: 50% Voxel size: 1.1 mm x 1.1 mm x 1.1 mm Geometrical analysis was then used on the raw data axials with certain measurement techniques to measure axial scoliotic rotation. Two observers, and one digital software measurement package (Osiris University of Geneva, Switzerland) was used to perform measurements on the axial scans. Background of geometrical analysis The imaging of AIS in the axial plane is of high importance, not only as a prognostic factor, but also for the accurate surgical correction of the scoliotic curve such as for safe pedicle screw placement. 10 Morphometric data gathered can reveal a great deal of information concerning the dimensions of pedicles, demon-

3 Magnetic resonance of adolescent idiopathic scoliosis The Radiographer 9 Figure 2 (a) Custom protocol Figure 2 (b) Custom protocol Figure 2 (c) Custom protocol Figure 2 (d) Custom protocol Figure 2 (e) Custom protocol Figure 3 (a) Custom protocol Figure 3 (b) Custom protocol Figure 3(c) Custom protocol Figure 3(d) Custom protocol strating the importance of accurate pre-operative planning and imaging. Basic outline of measurement analysis: axial end plates To perform the geometrical analysis, three sample data sets on the MRI axial images were taken for a set group of measurements. The measurements for the geometrical analysis of each axial endplate had been selected from texts including those used by Vaccaro et al. 10 and Maruta et al. 11 Each of these measurements would form a protocol to provide reliable anatomical parameters for the placement of pedicle screws before surgical intervention. The proposed protocol is listed below: chord length measured from the posterior cortical entry point of the pedicle to the anterior vertebral cortex in line with the axis of the pedicle. the length of the pedicle measured from the posterior cortex of the pedicle to the posterior longitudinal ligament along the axis of the pedicle. the transverse diameter (the medial-lateral outer cortical width of the pedicle), measured at the isthmus. the angle and insertion of the pedicle measured from the mid-line to the mid-axis of the pedicle. Additional measurements for analysis were also performed: the mid-point of the longest diameter through the vertebral body distance from spinous process to the posterior component of vertebral canal width of line connecting both laminae Figure 1 shows the measurement protocol used and how it was applied. The data were collected from three sources. They included two manual measurements Observer 1 and Observer 2, and one digital measurement. Examples of the measurements recorded on the images by the observers are demonstrated above in Figures 2(a) to 2(e) Measurements recorded using the Osiris software package are also displayed above in 3(a) to 3(d). A study proforma was used to record all data measurements. All measurements taken would include the mean, range and Figure 4 Study proforma used for tabulating data standard deviation (Figure 4 above). The significant result to draw from this statistical data would be the worth of the protocol as a trusted, reliable tool for pre-surgical measurement. While statistical analysis should show little variance in recorded measurements, it is the interpretation and subjectivity of each observer to understand the true position of the measurements, which will determine just how useful this method can possibly be. This is particularly important when used in the clinical setting among various surgeons. Any apparent variance is cause for refinement in subsequent studies of the performance of this protocol. 3D Reconstructions Siemens applications MRI coronal MPRs In this section, the multi-planar abilities of the MRI technique were explored. A comparison of this coronal MRI imaging against general radiography was made. Cobb Angle measurements were made on the coronal images using the same technique which was performed on the subject s weight-bearing AP radiographs. This would provide measure of the accuracy of the coronal MRI image. In gaining this measure, thought would need to be given to inherent limitations of the MRI technique in simulating the force of gravity on the scoliotic curve.

4 10 The Radiographer Cobb Angle measurements accurately represent the magnitude of a patient s scoliotic curve. The end-vertebrae marks the outer limits of the curve. It is defined as the endplate which meets at a parallel disc space. 3 After the upper and lower end vertebrae had both been identified, the curves were measured using the Cobb-Lippman technique. 12,13 This technique has proven very accurate with a variance of no more then three degrees between different examiners. 11 Results The following tables 1 5 display the data collection for five subjects with AIS. Each subject was identified as Subject and each axial was labelled as A, B and C. Discussion Findings: statistical evaluation and summary of measured variables axial vertebral rotation measurements Interpretation of plain film radiographs can sometimes be difficult in AIS, for example in the case of severe curves or congenital malformations. 14 The following section deals with the analysis of the cross sectional MRI data and evaluates the accuracy of the measurement protocol between three observers. The evaluation of each parameter was performed by using the standard deviation (σ) as the most reliable indicator of accuracy. A threshold accuracy of 2.5 standard deviations was used. Any variations outside this threshold were deemed excessive and highlighted in bold font in the tabulated data. After analysing the results, the following frequencies of errors (%) were found for each variable: Also, it would be worthwhile highlighting four (4) axial images where no variations were found: Subject 2000 Mid-curvature Subject 2000 End-curvature Subject 4000 End-curvature Subject 5000 Mid-curvature From the results, there are some clear trends that highlight parameters that have presented ambiguities among the three observers. The most significant being measurement A, measurement C, and measurement F. One possible reason for the variance could be the misunderstanding of measurements among observers. Other reasons could include ambiguous measurements within the protocol eg. angle measurements, poor visualisation of bony landmarks, images slightly oblique in the axial plane due to frontal or sagittal rotation, a lack of a sufficient number of subjects to average results, and errors in adjusting measurements due to the scaling of images on hard copy films. There may be a requirement to then alter the axial vertebral rotation protocol to eliminate those measurements which lead to Table 6 Measurement Σ Variations > 2.5 σ Chord length (A) 8/15 or 53.3% Length of the pedicle (D) 1/15 or 6.6% Transverse diameter (B) 0 Angle and insertion of the pedicle (C) 5/15 or 30% Middle of the longest diameter through vertebral body (E) 4/15 or 26.6% Distance from spinous process to posterior component of vertebral canal (F) 5/15 or 30% Width of line connecting both laminae 0 Table 1 Subject 1000 A Observer 1 Observer 2 Digital Reading Mean Range SD A D B C E F G Subject 1000 B Observer 1 Observer 2 Digital Reading Mean Range SD A D B C E F G Subject 1000 C Observer 1 Observer 2 Digital Reading Mean Range SD A D B C E F G

5 Magnetic resonance of adolescent idiopathic scoliosis The Radiographer 11 Table 2 Subject 2000 A Observer 1 Observer 2 Digital Reading Mean Range SD A D B C E F G Subject 2000 B Observer 1 Observer 2 Digital Reading Mean Range SD A D B C E F G Subject 2000 C Observer 1 Observer 2 Digital Reading Mean Range SD A D B C E F G Table 3 Subject 3000 A Observer 1 Observer 2 Digital Reading Mean Range SD A D B C E F G Subject 3000 B Observer 1 Observer 2 Digital Reading Mean Range SD A D B C E F G Subject 3000 C Observer 1 Observer 2 Digital Reading Mean Range SD A D B C E F G

6 12 The Radiographer Table 4 Subject 4000 A Observer 1 Observer 2 Digital Reading Mean Range SD A D B C E F G Subject 4000 B Observer 1 Observer 2 Digital Reading Mean Range SD A D B C E F G Subject 4000 C Observer 1 Observer 2 Digital Reading Mean Range SD A D B C E F G excessive variations among observers. Alternatively, particular measurements such as the chord length may need to be more clearly defined to the observer. What the study has done though, is to briefly demonstrate reliable and practical elements of a pre-surgical protocol that could be practically applied. Findings: 3D coronal reconstructions Analysis of the Cobb Angle measurements of MRI coronal MPRs compared to that of the AP weight-bearing radiograph measurements delivered several results. MRI measurements showed angles that were mostly lower than that of the X-ray measurements. Also, there were some identifiable trends between both readings indicating that there is a correlation between both modalities in angle assessment. Figure 5 shows two examples of coronal MRI images acquired during the study. Figure 6 is the graphical display of the relative Cobb angles on the same patients for MRI and general radiography. The lower values in MRI measurements suggest a definite change in the nature of the curve in the supine position compared to the erect position. MRI in the standing position is possible on certain modern MRI scanners. The trends identified in these results suggest that, for MRI to be a viable option for the assessment of the magnitude of the curve, it will be necessary to simulate the force of gravity from the standing position on the spine in the supine position for a reliable result. This option would require further research in a much larger scale than can be attempted in this paper. Currently the Mater Spinal Research Centre in Brisbane is conducting work in this field on possible applications. This includes the use of a pneumatic chamber to apply compression to the spine at the correct force (N) Angle 25 -Ray 20 MRI Figure 5 (a) Custom protocol Figure 5 (b) Subject No. Figure 6 Graphical representation showing relative differences between the Cobb Angle results of X-ray and MRI

7 Magnetic resonance of adolescent idiopathic scoliosis The Radiographer 13 Table 5 Subject 5000 A Observer 1 Observer 2 Digital Reading Mean Range SD A D B C E F G Subject 5000 B Observer 1 Observer 2 Digital Reading Mean Range SD A D B C E F G Subject 5000 C Observer 1 Observer 2 Digital Reading Mean Range SD A D B C E F G Recommendations To further investigate the design of a pre-surgical measurement protocol involving parameters that represent the least possible statistical variance. Axial rotation must be assessed using reliable datum points that have been shown to give minimal interobserver variation. This protocol must contain parameters that are clearly defined, that lack ambiguity, and have a low subjectivity amongst observers. To follow-up and assess the clinical benefits of such a protocol with spinal specialists and orthopaedic surgeons alike. To pursue research in the area of Cobb Angle assessment of the AIS patient in the supine position, with the use of pneumatic chambers and bracing devices. Conclusion This paper has endeavoured to justify the use of MRI in the routine assessment of adolescent idiopathic scoliosis. It can be shown that MRI is indeed a feasible imaging option. The application of MRI in the treatment of AIS shows great promise. With the absence of ionising radiation, it allows safe, reliable and accurate imaging. Follow-up imaging is also much more clinically justified than CT, though susceptibility artefact may be an issue with metallic surgical implants. However, CT retains dominance in regard to factors such as speed and three-dimensional imaging and remains a popular choice for referring physicians and surgeons alike. But the underlying issue is ionising radiation dose. The cumulative dose received from repeat examinations in monitoring progression or response to treatment, should minimise the use of CT as a means of follow-up for scoliotic patients in an adolescent age group. General radiography follow-up of AIS is preferred by specialists as they are fast, inexpensive and accurate for this application. This study has shown that, with an appropriate protocol, the MRI technique can be used successfully to evaluate geometrical aspects of the adolescent spine. This project has stressed the features of the MRI modality and highlighted its capabilities in geometrical assessment and threedimensional reconstruction. Refined protocols for the assessment of geometrical rotation, that consider the measurement variations above, should be recommended for adoption. Three-dimensional coronal imaging reveals important information about the nature of the scoliotic curve with accuracy. In the future, research into MRI will continue to pursue applications such as vertebral morphology and disc disease in AIS patients. MRI is a modality with such an exciting future, and its clinical usefulness is constantly being explored. Considering the accuracy of MRI in assessing AIS demonstrated in this project, the modality, despite its small number of disadvantages, may well become the viable option of choice for the pre-surgical assessment of AIS. References 1 Dickson RA. Spinal Deformity Basic Principles. Curr Orthop 2004; 18: Reamy BV, Slakey JB. Adolescent Idiopathic Scoliosis: Review and Current Concepts. Am Fam Physician 2001; 64: Redla S, Sikadar T, Saifuddin A. Magnetic Resonance Imaging of Scoliosis. Clin Radiol 2001; 56: Bushong SC. Radiologic Science for Technologists 6th Edition 1997 USA: Mosby. 5 Perie D, Sales de Gauzy J, Curnier D, Hobatho MC. Intervertebral Disc Modelling using a MRI Method: Migration of the Nucleus Zone within

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