Clinical evidence Spectral CT in musculoskeletal disorders

David Maintz, MD, Department of Diagnostic and Interventional Radiology, University Hospital of Cologne, Cologne, Germany

Victor Neuhaus, MD, Department of Diagnostic and Interventional Radiology, University Hospital of Cologne, Cologne, Germany

Introduction

Musculoskeletal imaging in conventional CT suffers from low contrast of several soft tissues, the lack of visibility of bone marrow changes, and artifacts caused by metal implants. The strength of CT in regard to musculoskeletal imaging is the assessment of cortical and trabecular bone. The value of the new imaging qualities of spectral CT as investigated so far is described in this chapter.

Bone

Bone mineral density measurements

Osteoporosis is a widespread disease in elderly men and women. It leads to a pathologic reduction of bone strength which results in fractures, hereby causing severe pain, immobility, neurologic symptoms, and increased mortality.1,2 The surrogate parameter for the detection of reduced bone strength is the measurement of bone mineral density in different skeletal regions such as the spine, the hip, and the forearm. Dual-energy X-ray absorptiometry (DXA) is the standard device for bone mineral density (BMD) measurements, detecting areal BMD from low dose dual-energy X-ray data. For the individual patient, a measured T-score (a standardized score, comparing BMD to the average value of a 30-year-old healthy adult) of -2.5 or less is defined as osteoporosis by the World Health Organization.3 However, DXA suffers from several limitations such as overlap of calcified plaques and measurement errors due to fat tissue. As an alternative to DXA, Quantitative Computed Tomography (QCT) is able to perform three-dimensional volumetric bone mineral density measurements in the targeted bone tissues. QCT commonly uses a calibration phantom, which is scanned synchronous or asynchronous with the patient in order to allow for a precise volumetric BMD (vBMD) estimation over several time points and different scanners.4 However, QCT also suffers from partial volume effects and inhomogeneous bone marrow, which influences the results of the measurements.5

With regard to BMD measurements, the dual-energy CT data acquired with a spectral detector CT (SDCT) allow for a material decomposition similar to DXA, which is considered to be more robust due to the 3D spatial resolution. Furthermore, the dual-energy CT data might allow for an equally accurate calibration in comparison to phantom-based QCT, and the anticipated ability of SDCT to identify calcium-hydroxyapatite might improve precision of vBMD estimation significantly.

In the few existing phantom-based studies, spectral CT data derived from SDCT proved to allow for an accurate quantification of BMD using different approaches for differentiation of calcium-hydroxyapatite.6,7 In one study, BMD quantification was performed using mass attenuation coefficients across different virtual monoenergetic levels,6 while in the other study by Mei and colleagues, calibration measurements were performed and thus attenuation profiles specific for the scanner used in that study were acquired.7 In both studies, SDCT vBMD values of calcium-hydroxyapatite specific BMD were compared to DXA and QCT showing close correlation.6,7 In addition to the known advantages of measuring BMD in CT in comparison to DXA, the measurement of BMD in SDCT appears to have distinct advantages in comparison to QCT. 7 First, vBMD measured in obese patients should not be affected by beam hardening since material decomposition is performed in the projection space, and thus beam hardening artifacts are corrected.6-9 Second, calcium-hydroxyapatite specific measurements of vBMD should be less susceptible to other materials affecting attenuation such as fat and iodine.7 Correspondingly, vBMD measured in QCT was found to be higher than vBMD measured in SDCT, which may indicate a more accurate measurement by SDCT. 7

In SDCT, the measurements of vBMD can be performed retrospectively since they do not require an additional phantom. Further, different scanning protocols applying different tube voltages and tube currents can be used to acquire the dual-energy CT data, enabling accurate opportunistic screening for osteoporosis in CT scans performed for different clinical indications.6,7 Since the above-mentioned studies were ex vivo and conducted on phantoms, which contained only calcium-hydroxyapatite and water, further patient-based studies are required to confirm these investigations in vivo and to evaluate applicability of the proposed material decomposition in the presence of diverse confounding materials.

Detection of traumatic bone marrow changes

While CT is the method of choice for the accurate three-dimensional depiction of bone structure in clinical imaging, providing an excellent depiction of cortical and trabecular bone, magnetic resonance imaging is superior to CT for visualization of bone marrow changes. Fluid-sensitive sequences combined with T1-weighted sequences allow the diagnosis of bone marrow edema, while in conventional CT, the bone marrow is obscured by the trabecular bone.10,11 As SDCT allows for a material decomposition (e.g., iodine, fat, and calcium), virtual material maps can be reconstructed, in which these materials are subtracted or enhanced. With regard to bone, these opportunities of SDCT appear clinically relevant. The material maps could be used to diagnose bone marrow edema.

Calcium suppressed (CaSupp) images are SDCT reconstructions in which the identified virtual calcium component has been subtracted reducing cortical and trabecular bone obscuring the bone marrow. In this condition, diffuse bone marrow changes which are normally not visible in conventional CT can be assessed (Figure 1). Visualization of bone marrow changes can either be achieved by using a material decomposition process, in which the spectral based images (SBI) are converted into sets of dynamic material pairs (soft tissue and a material with a user-defined level of calcium composition that is indicated by the calcium suppression index (CSI) value enabling reconstruction of calcium suppressed images) or by three material decomposition (enabling reconstruction of red marrow maps, yellow marrow maps, and calcium-hydroxyapatite maps.)12,13 In opposition to the technical approaches to dual-energy CT offered by other vendors, the extent of calcium subtraction in CaSupp images can be modified in order to target varying calcium densities and to achieve optimal contrast of bone marrow edema. Therefore, different calcium suppression indices (CSI) can be chosen while reading the images. Here, CSI can be adjusted on a scale from 25 to 100; a high CSI targets tissues with a high calcium composition weight, while a low CSI targets tissues with a low calcium composition weight.

The possibility to adjust CSI while reading the images appears to offer a certain advantage, since bone marrow edema adjacent to the endplates of the vertebrae (which can be obscured by the denser cortical bone) can be visualized by choosing slightly lower CSI of 70 and 80 in contrast to 90 and 100, which depict bone marrow edema within the trabecular bone compartment. Primary studies investigating CaSupp as well as three material decomposition imaging showed a high sensitivity and specificity for the detection of traumatic bone marrow edema in vertebral compression fractures in correlation with MRI.12,13 Furthermore, the investigated high negative predictive value emphasizes the potential of CaSupp to rule out bone marrow edema due to acute vertebral fractures. In this regard, there was an excellent inter-rater agreement reported, indicating a robust clinical applicability of these maps. Finally, CaSupp might allow detection of occult fractures since bone marrow edema was also detected in vertebrae without obvious signs of fractures in conventional CT. Thus, SDCT might allow a reduction in the number of MRI examinations needed in order to differentiate acute from older fractures.

84-year-old male patient who underwent SDCT of the lumbar spine due to severe lower back pain. Grayscale conventional CT images (A) show four fractured thoracolumbar vertebrae. Additional MRI confirmed bone marrow edema in lumbar vertebrae 3, 4, and 5 in fluid sensitive STIR sequence (B, marked in red) and T1-weighted sequence (C). CaSupp images (D) show bone marrow edema in lumbar vertebrae 3, 4, and 5 (marked in red) in correlation to the MRI. In addition, CaSupp images rule out acute fracture of thoracic vertebra 12 and lumbar vertebra 1, which showed a fracture on conventional CT, but no bone marrow edema in MRI and CaSupp images (marked in blue).

Detection of bone marrow changes due to malignant lesions

Bone metastases in cancer patients are associated with poor prognosis and skeletal complications. Thus, an early detection of bone metastases is required in order to provide appropriate treatment.14 However, assessment of bone lesion in conventional CT is limited to size, margin, location, and density of the lesion itself and the surrounding bone. Therefore, using CT alone, bone lesions might remain undiagnosed or unclear in regard to their etiology. Thus, more expensive additional diagnostic modalities are frequently being performed such as positron emission tomography or bone scintigraphy, in order to enhance the detection of bone lesions and their etiologic characteristics.15 In the first studies, SDCT has been found to enable differentiation of bone metastases and normal bone as well as to enable visualization of bone marrow changes adjacent to bone lesions in CaSupp images.16,17 Furthermore, iodine quantification has been investigated as a promising parameter for the separation of vertebral trabecular bone metastases and healthy trabecular bone.18 Detection of bone marrow changes as well as increased iodine uptake adjacent to vertebral lesion might allow earlier detection and accurate classification of otherwise unclear vertebral lesions. Additionally, this might spare additional imaging tools such as positron emission tomography or bone scintigraphy. However, for these purposes, further structured investigations are needed.

Imaging of metal implants and surrounding tissues

The increasing prevalence of orthopedic metal implants is a challenge for diagnostic imaging, especially in computed tomography, since image quality and diagnostic accuracy of CT might be severely impaired by metal artifacts. Artifacts are caused by complete absorption of the photons of the X-ray (photon starvation) and increased attenuation of the low energy photons in comparison to the high energy photons of the X-ray (beam hardening).19,20 In general, the severity of the artifacts is influenced by the thickness of the metal implants as well as the alloy, of which the metal implants are made.21,22 As shown in ex vivo phantombased studies as well as in vivo patient-based studies for metal artifacts, in general, virtual monoenergetic images reconstructed from SDCT are a powerful tool in order to improve image quality around implants.21-23 Virtual monoenergetic images at high keV reduce beam hardening artifacts and moderate photon starvation artifacts without leading to significant additional artifacts, and thus improve the assessment of the bone adjacent to metal implants as well as the implant itself.21-23

With regard to smaller metal implants in the spine or in the extremities, virtual monoenergetic images at high keV can completely remove artifacts.22,23 Also, moderate artifacts due to total arthroplasties of the hips can be reduced significantly using virtual monoenergetic images.21,23 However, severe artifacts, especially photon starvation artifacts caused by unilateral or bilateral total arthroplasties of the hip, could not be reduced to a satisfactory extent (Figure 2).21,23 Therefore, it is of dedicated interest that current studies find different strengths of virtual monoenergetic images as well as iterative algorithms dedicated for orthopedic metal artifact reduction.22,24 Here, first results indicate a benefit for metal artifact reduction when using a combination of iterative metal artifact reduction and virtual monoenergetic images in cases of uni- and bilateral total arthroplasties of the hip, especially in case of severe photon starvation artifacts overlaying the pelvic organs (Figure 2).

71-year-old male patient who received a staging CT due to esophageal cancer. Conventional CT images (A) show severe artifacts caused by bilateral total arthroplasty of the hips. On virtual monoenergetic images at high keV (B and C), those artifacts are only slightly reduced. Conventional CT images post-processed with a dedicated algorithm (OMAR) (D) show improved visibility of the intrapelvic structures. However, artifacts close to the implants remain visible. The combination of virtual monoenergetic images at high keV and the dedicated algorithm (E and F) demonstrate nearly complete removal of the artifacts, as well as significantly improved assessment of adjacent and intrapelvic structures.

Soft tissue

Visualization of intervertebral discs

Visualization of soft tissue adjacent or in between bones, especially intervertebral discs, is limited in conventional CT, and thus, in contrast to MRI, the diagnosis of herniated intervertebral discs remains challenging in conventional CT. In a case review study, the use of CaSupp in imaging of the intervertebral discs has been found to be beneficial, since the suppression of the vertebral bone increases visibility of the intervertebral discs.25 This might enable improved detection of pathologies such as herniated discs. However, further research is needed to confirm these preliminary findings.

Imaging of intraspinal metastases

Similar to pathologies of the intervertebral discs, the imaging of intraspinal metastases can be challenging in conventional CT due to the low contrast between intraspinal masses and the cerebrospinal fluid. However, conventional CT is the modality of choice used to detect advanced stage of cancer. Thus, additional MRI is required to assess intraspinal metastases. Accurate assessment of intraspinal metastases is needed because these metastases might cause neurologic symptoms and require surgical resection. The depiction of intraspinal metastases due to malignancies is improved in virtual monoenergetic images at low keV, which offer increased iodine attenuation, while image noise remains fairly low, and thus increases visibility as well as demarcation towards cerebrospinal fluid as well as epidural fat (Figure 3). While some intraspinal metastases cannot be detected or are difficult to assess in conventional CT, virtual monoenergetic images reconstructed at low keV allow easy assessment and detection of spinal metastases in the spinal canal.26 Thus, imaging of intraspinal metastases using MRI might not be necessary in all cases of intraspinal metastases, and virtual monoenergetic images might allow earlier detection in staging CT before clinical symptoms due to spinal stenosis occur.

Imaging of intramuscular metastases

Skeletal muscle metastases are often missed in staging CTs of oncologic patients. This is due to the fact that other organs or anatomic regions demand more attention by radiologists since skeletal muscle metastases are rare in the majority of cancer patients. Additionally, contrast between muscle tissue and the metastases is usually low. Iodine overlay images reconstructed from SDCT improve the detection of intramuscular metastases (Figure 4). In a patient-based study, sensitivity for skeletal muscle metastases by malignant melanoma has been found to be significantly improved in iodine overlay images, while specificity remains constant.27

Future applications in musculoskeletal imaging

While several advantages of SDCT over conventional CT in musculoskeletal imaging have already been investigated, many more clinical applications will follow since SDCT has just been recently introduced to clinical routine. Moreover, some applications of dual-energy CT in musculoskeletal imaging have been discovered for other approaches to dual-energy CT (e.g., detection of gout tophi or visualization of collagenous structures such as ligaments or tendons).28

78-year-old female patient diagnosed with metastatic lung cancer (A, upper row), and a 77-year-old male patient diagnosed with metastatic prostate cancer (B, lower row), who received a staging CT. In patient A, conventional images show an osseous metastasis, which is partially localized within the spinal canal. In patient B, conventional images revealed an obliteration of epidural fat within the spinal canal due to suspected intraspinal metastasis. MRI (contrast-enhanced, fat-saturated T1-weighted sequence) and virtual monoenergetic images (VMI) at 40 keV both clearly depict intraspinal tumor growth in patient A and intraspinal spreading in patient B. Note that the contrast and delineation toward the cerebrospinal fluid and the spinal content is significantly improved in VMI at 40 keV in contrast to conventional CT images.

35-year-old male patient who received a staging CT due to metastatic malignant melanoma. Metastases to the right (A) and left (B) musculus gluteus maximus as well as to the paravertebral muscles (C and D) are difficult to detect in conventional CT (red arrows), and in some cases, might have remained undiagnosed; while in iodine overlay images (lower row), those metastases (red arrows) are easy to detect due to improved contrast to surrounding normal muscle tissue.

References

1. Fink HA, Ensrud KE, Nelson DB, et al. Disability after clinical fracture in postmenopausal women with low bone density: the fracture intervention trial (FIT). Osteoporos Int. 2003 Jan;14(1):69-76.

2. Bliuc D, Nguyen ND, Milch VE, Nguyen TV, Eisman JA, Center JR. Mortality risk associated with low-trauma osteoporotic fracture and subsequent fracture in men and women. JAMA. 2009 Feb 4;301(5):513-21.

3. Prevention and management of osteoporosis. World Health Organ Tech Rep Ser. 2003;921:1-164, back cover.

4. Engelke K, Lang T, Khosla S, et al. Clinical Use of Quantitative Computed Tomography-Based Advanced Techniques in the Management of Osteoporosis in Adults: the 2015 ISCD Official Positions-Part III. J Clin Densitom. 2015 Jul-Sep;18(3):393-407.

5. Kuiper JW, van Kuijk C, Grashuis JL, Ederveen AG, Schütte HE. Accuracy and the influence of marrow fat on quantitative CT and dual-energy X-ray absorptiometry measurements of the femoral neck in vitro. Osteoporos Int. 1996;6(1):25-30.

6. van Hamersvelt RW, Schilham AMR, Engelke K, et al. Accuracy of bone mineral density quantification using dual-layer spectral detector CT: a phantom study. Eur Radiol. 2017 Oct; 27(10):4351-4359.

7. Mei K, Schwaiger BJ, Kopp FK, et al. Bone mineral density measurements in vertebral specimens and phantoms using dual-layer spectral computed tomography. Sci Rep. 2017 Dec 13;7(1):17519.

8. Maass C, Baer M, Kachelriess M. Image-based dual energy CT using optimized precorrection functions: a practical new approach of material decomposition in image domain. Med Phys. 2009 Aug;36(8):3818-29.

9. Kalender WA, Klotz E, Kostaridou L. An algorithm for noise suppression in dual energy CT material density images. IEEE Trans Med Imaging. 1988;7(3):218-24.

10. Lenchik L, Rogers LF, Delmas PD, Genant HK. Diagnosis of osteoporotic vertebral fractures: importance of recognition and description by radiologists. AJR Am J Roentgenol. 2004 Oct;183(4):949-58.

11. Uppin AA, Hirsch JA, Centenera LV, et al. Occurrence of new vertebral body fractures after percutaneous vertebroplasty in patients with osteoporosis. Radiology. 2003 Jan;226(1):119-24.

12. Schwaiger BJ, Gersing AS, Hammel J, et al. Three-material decomposition with dual-layer spectral CT compared to MRI for the detection of bone marrow edema in patients with acute vertebral fractures. Skeletal Radiol. 2018 May 25.

13. Neuhaus V, Lennartz S, Abdullayev N, et al. Bone marrow edema in traumatic vertebral compression fractures: Diagnostic accuracy of dual-layer detector CT using calcium suppressed images. Eur J Radiol. https://doi.org/10.1016/j.ejrad.2018.06.009.

14. Reddi AH, Roodman D, Freeman C, Mohla S. Mechanisms of Tumor Metastasis to the Bone: Challenges and Opportunities. J Bone Miner Res. 2003 Feb;18(2):190-4.

15. Rybak LD, Rosenthal DI. Radiological imaging for the diagnosis of bone metastases. Q J Nucl Med 2001; 45:53–64.

16. Abdullayev N, Romman Z, Pahn G, Shapira N, Kafri G. Visualizing bone marrow in the presence of bone lesion with calcium-suupressed CT images. Available from: http://clinical.netforum. healthcare.philips.com/global/Explore/Case-Studies/CT/Visualizing-bone-marrow-in-thepresence-of-bone-lesions-with-calcium-suppressed-CT-images.

17. Grosse Hokamp N, Abdullayev N, Neuhaus V, Holz J, Maintz D, Borggrefe J. Calcium suppression in spectral detector computed tomography improves visualization of bone metastasis. Presented at ECR 2018.

18. Abdullayev N, et al. Spectral Detector Computed Tomography Iodine Density Thresholds for the Separation of Vertebral Bone Metastases. submitted to Investigative Radiology.

19. Mori I, Machida Y, Osanai M, Iinuma K. Photon starvation artifacts of X-ray CT: their true cause and a solution. Radiol Phys Technol. 2013 Jan;6(1):130-41.

20. Brooks RA, Di Chiro G. Beam hardening in x-ray reconstructive tomography. Phys Med Biol. 1976 May;21(3):390-8.

21. Wellenberg RH, Boomsma MF, van Osch JA, et al. Quantifying metal artefact reduction using virtual monochromatic dual-layer detector spectral CT imaging in unilateral and bilateral total hip prostheses. Eur J Radiol. 2017 Mar;88:61-70.

22. Große Hokamp N, Neuhaus V, Abdullayev N, et la. Reduction of artifacts caused by orthopedic hardware in the spine in spectral detector CT examinations using virtual monoenergetic image reconstructions and metal-artifact-reduction algorithms. Skeletal Radiol. 2018 Feb;47(2):195-201.

23. Neuhaus V, Große Hokamp N, Abdullayev N, et al. Metal artifact reduction by dual-layer computed tomography using virtual monoenergetic images. Eur J Radiol. 2017 Aug;93:143-148.

24. Laukamp KR, Lennartz S, Neuhaus VF, et al. CT metal artifacts in patients with total hip replacements: for artifact reduction monoenergetic reconstructions and post-processing algorithms are both efficient but not similar. Eur Radiol. 2018 May 3.

25. Lennartz S, Romman Z, Pahn G, Shapira N, Kafri G. Visualizing intervertebral disc herniation with calcium-suppressed CT images. Available from: http://clinical.netforum.healthcare.philips.com/ us_en/Explore/Case-Studies/CT/Visualizing-intervertebral-disc-herniation-with-calciumsuppressed-CT-images.

26. Abdullayev N, Maus V, Kiel A, et al. Improved detection of intraspinal metastases in dual-layer detector CT: Diagnostic accuracy of virtual monoenergetic reconstructions compared with polyenergetic reconstructions. Unpublished.

27. Lennartz S, Le Blanc M, Abdullayev N, et al. Improved detection of skeletal muscle metastases in iodine-density overlay maps and virtual monoenergetic reconstructions provided by spectral detector CT. Presented at ECR 2018.

28. Mallinson PI, Coupal TM, McLaughlin PD, Nicolaou S, Munk PL, Ouellette HA. Dual-Energy CT for the Musculoskeletal System. Radiology. 2016 Dec;281(3):690-707.

History

Benefits or pitfalls of dual-energy CT

Key images

Findings

62-year-old female with left hip prosthesis presented with hip pain. Decreased metal artifact at higher keV virtual monoenergetic images.

Axial images

Extensive metal artifacts from hip prosthesis were seen on conventional CT images. This made evaluation of the periprosthetic bone and adjacent soft tissue challenging. The artifacts were significantly lower at virtual monoenergetic spectral images at 200 keV.

Discussion

Virtual monoenergetic imaging at higher keV could significantly reduce metal artifacts and improve diagnostic image quality at periprosthetic areas.

History Benefits or pitfalls of dual-energy CT

Key images

Findings