Quantitative computed tomography

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Quantitative computed tomography
ICD-9 88.98

Quantitative computed tomography (QCT) is a medical technique that measures bone mineral density (BMD) using a standard X-ray computed tomography (CT) scanner with a calibration standard to convert Hounsfield units (HU) of the CT image to bone mineral density values. [1] Quantitative CT scans are primarily used to evaluate bone mineral density at the lumbar spine and hip.

Contents

In general, solid phantoms placed in a pad under the patient during CT image acquisition are used for calibration. These phantoms contain materials that represent a number of different equivalent bone mineral densities. Usually either calcium hydroxyapatite (CaHAP) or potassium phosphate (K2HPO4) are used as the reference standard. [2]

Image of cortical and trabecular bone of the spine by Quantitative computed tomography. Only the central trabecular portion is measured Image of trabecular bone of the spine by Quantitative computed tomography.jpg
Image of cortical and trabecular bone of the spine by Quantitative computed tomography. Only the central trabecular portion is measured

History

QCT was invented at University of California San Francisco (UCSF) during the 1970s. Douglas Boyd, PhD and Harry Genant, MD used a CT head scanner to do some of the seminal work on QCT. [3] At the same time, CT imaging technology progressed rapidly and Genant and Boyd worked with one of EMI's first whole body CT systems in the late 1970s and early 1980s to apply the quantitative CT method to the spine, coining the term "QCT." Genant later published several articles on spinal QCT in the early 1980s with Christopher E. Cann, PhD. Today, QCT is being used in hundreds of medical imaging centers around the world, both clinically and as a powerful research tool.

Three-dimensional QCT imaging

Originally, conventional 2D QCT used individual, thick CT slice images through each of multiple vertebrae which involved tilting the CT scanner gantry to align the slice with each vertebra. Today, modern 3D QCT uses the ability of CT scanners to rapidly acquire multiple slices to construct three-dimensional images of the human body. Using 3D imaging substantially reduced image acquisition time, improved reproducibility and enabled QCT bone density analysis of the hip. [1]

Image of 3D volumetric QCT scan Image of 3D volumetric QCT scan.jpg
Image of 3D volumetric QCT scan

Diagnostic use

QCT exams are typically used in the diagnosis and monitoring of osteoporosis.

Lumbar spine

At the spine, QCT is used to measure the bone mineral density of only the spongy interior bone separately from the dense cortical bone that forms the exterior walls of the vertebrae. [4] The trabecular bone has much higher metabolic activity than the cortical bone and so is affected by age, disease and therapy-related changes earlier and to a greater degree than cortical bone. This means that QCT of the spine has an advantage compared to other bone density tests because earlier changes in bone mineral density may be detected . [1]

Hip

Image of proximal femur bone projection Image of proximal femur bone projection.jpg
Image of proximal femur bone projection

Clinically, QCT is used at the hip to produce areal BMD measurements and T-Scores that are equivalent to DXA measurements. [5] The exam can be done without particular attention to the positioning of the patient's limbs because the software allows the hip anatomy to be manipulated after the image is captured, allowing the exam to be performed on patients with arthritic hips who may find traditional exams uncomfortable.

Contraindications for use

QCT bone densitometry should not be used for patients who have the following conditions:

Radiation dose

QCT scan protocols are low-dose and can limit the amount of radiation exposure to between 200-400μSv for a spine exam [6] This is comparable to a set of mammograms and typically substantially less than a standard CT exam. Using other non-IV contrast abdominal or pelvic scans such as a Virtual Colonography studies, the QCT exam can be performed without requiring any further image acquisition or consequent radiation dose to the patient. [7]

Advantages

QCT enables spine BMD measurements on patients with scoliosis, which cannot usually be measured using Dual-energy X-ray absorptiometry (DXA). [8] In addition, QCT can avoid the artificially high BMD measurements that can confuse the results from DXA in arthritic patients, patients who are obese, [9] who suffer from disc space narrowing or spinal degenerative diseases, [10] aortic calcification [11] or osteophytes. [12]

Reproducibility

Short-term precision estimates of BMD measurement by 3D QCT have been published for the lumbar spine as 0.8% [13] and femoral neck as 0.69%. [5]

Dual use of CT images

Several studies have shown that bone density may be measured by QCT using CT images that were ordered for other purposes. Using pre-existing images, including CT colonography exams, [14] QCT allows for bone density screening without submitting the patient to any additional radiation exposure. The feasibility of using routine abdominal contrast-enhanced CT scans for the evaluation of bone density by QCT has also been demonstrated. [15]

Reporting

Average bone mineral density is calculated and then compared to age and sex matched controls. At the spine, a volumetric BMD measurement is made using QCT and rather than using T-Scores, it should be compared to guideline thresholds from the American College of Radiology (ACR): [16] a BMD < 80 mg/cm3 indicates osteoporosis; a BMD < 120 mg/cm3 and > 80 mg/cm3 indicates osteopenia; and a BMD above 120 mg/cm3 is considered normal.

At the hip, a DXA-equivalent T-score may be calculated for comparison to the WHO classification at the proximal femur as normal, osteopenia (T-Score < -1.0 and > -2.5) or osteoporosis (T-Score < -2.5). [17] This T-Score may also be used for fracture risk probability calculation in the WHO FRAX tool [18] with "T-Score" as the appropriate DXA setting.

Peripheral quantitative computed tomography

In medicine, peripheral quantitative computed tomography , commonly abbreviated pQCT, is a type of quantitative computed tomography (QCT), used for making measurements of the bone mineral density (BMD) in a peripheral part of the body, such as the forearms or legs as opposed to QCT that measures bone mineral density at the hip and spine. It is useful for measuring bone strength. [19]

Comparison to DXA

Unlike most other common techniques for measuring BMD, a pQCT scan is able to measure volumetric bone mineral density, plus other measures such as the stress-strain index (SSI) and the geometry of the bone. DXA is only able to provide the areal bone mineral density.

High-resolution peripheral quantitative computed tomography (HR-pQCT) is better than DXA at detecting bone microarchitecture, modeling whole-bone geometry using 3-dimensional information from scans. This method allows estimation of bone strength and other mechanical properties. [20]

See also

Related Research Articles

<span class="mw-page-title-main">Osteoporosis</span> Skeletal disorder

Osteoporosis is a systemic skeletal disorder characterized by low bone mass, micro-architectural deterioration of bone tissue leading to bone sterility, and consequent increase in fracture risk. It is the most common reason for a broken bone among the elderly. Bones that commonly break include the vertebrae in the spine, the bones of the forearm, the wrist, and the hip. Until a broken bone occurs there are typically no symptoms. Bones may weaken to such a degree that a break may occur with minor stress or spontaneously. After the broken bone heals, the person may have chronic pain and a decreased ability to carry out normal activities.

<span class="mw-page-title-main">CT scan</span> Medical imaging procedure using X-rays to produce cross-sectional images

A computed tomography scan is a medical imaging technique used to obtain detailed internal images of the body. The personnel that perform CT scans are called radiographers or radiology technologists.

<span class="mw-page-title-main">Radiography</span> Imaging technique using ionizing and non-ionizing radiation

Radiography is an imaging technique using X-rays, gamma rays, or similar ionizing radiation and non-ionizing radiation to view the internal form of an object. Applications of radiography include medical and industrial radiography. Similar techniques are used in airport security,. To create an image in conventional radiography, a beam of X-rays is produced by an X-ray generator and it is projected towards the object. A certain amount of the X-rays or other radiation are absorbed by the object, dependent on the object's density and structural composition. The X-rays that pass through the object are captured behind the object by a detector. The generation of flat two-dimensional images by this technique is called projectional radiography. In computed tomography, an X-ray source and its associated detectors rotate around the subject, which itself moves through the conical X-ray beam produced. Any given point within the subject is crossed from many directions by many different beams at different times. Information regarding the attenuation of these beams is collated and subjected to computation to generate two-dimensional images on three planes which can be further processed to produce a three-dimensional image.

<span class="mw-page-title-main">Radiology</span> Branch of Medicine

Radiology is the medical discipline that uses medical imaging to diagnose diseases and guide their treatment, within the bodies of humans and other animals. It began with radiography, but today it includes all imaging modalities, including those that use no electromagnetic radiation, as well as others that do, such as computed tomography (CT), fluoroscopy, and nuclear medicine including positron emission tomography (PET). Interventional radiology is the performance of usually minimally invasive medical procedures with the guidance of imaging technologies such as those mentioned above.

<span class="mw-page-title-main">Dual-energy X-ray absorptiometry</span> Diagnostic test for bone mineral density testing

Dual-energy X-ray absorptiometry is a means of measuring bone mineral density (BMD) using spectral imaging. Two X-ray beams, with different energy levels, are aimed at the patient's bones. When soft tissue absorption is subtracted out, the bone mineral density (BMD) can be determined from the absorption of each beam by bone. Dual-energy X-ray absorptiometry is the most widely used and most thoroughly studied bone density measurement technology.

The Hounsfield scale, named after Sir Godfrey Hounsfield, is a quantitative scale for describing radiodensity. It is frequently used in CT scans, where its value is also termed CT number.

<span class="mw-page-title-main">Osteopenia</span> Medical condition

Osteopenia, known as "low bone mass" or "low bone density", is a condition in which bone mineral density is low. Because their bones are weaker, people with osteopenia may have a higher risk of fractures, and some people may go on to develop osteoporosis. In 2010, 43 million older adults in the US had osteopenia. Unlike osteoporosis, osteopenia does not usually cause symptoms, and losing bone density in itself does not cause pain.

<span class="mw-page-title-main">Bone density</span> Amount of bone mineral in bone tissue

Bone density, or bone mineral density, is the amount of bone mineral in bone tissue. The concept is of mass of mineral per volume of bone, although clinically it is measured by proxy according to optical density per square centimetre of bone surface upon imaging. Bone density measurement is used in clinical medicine as an indirect indicator of osteoporosis and fracture risk. It is measured by a procedure called densitometry, often performed in the radiology or nuclear medicine departments of hospitals or clinics. The measurement is painless and non-invasive and involves low radiation exposure. Measurements are most commonly made over the lumbar spine and over the upper part of the hip. The forearm may be scanned if the hip and lumbar spine are not accessible.

The stress–strain index (SSI), of a bone, is a surrogate measure of bone strength determined from a cross-sectional scan by QCT or pQCT. The stress–strain index is used to compare the structural parameters determined by analysis of QCT/pQCT cross-sectional scans to the results of three-point bending test.

Senile osteoporosis has been recently recognized as a geriatric syndrome with a particular pathophysiology. There are different classification of osteoporosis: primary, in which bone loss is a result of aging and secondary, in which bone loss occurs from various clinical and lifestyle factors. Primary, or involuntary osteoporosis, can further be classified into Type I or Type II. Type I refers to postmenopausal osteoporosis and is caused by the deficiency of estrogen. While senile osteoporosis is categorized as an involuntary, Type II, and primary osteoporosis, which affects both men and women over the age of 70 years. It is accompanied by vitamin D deficiency, body's failure to absorb calcium, and increased parathyroid hormone.

Steroid-induced osteoporosis is osteoporosis arising from the use of glucocorticoids analogous to Cushing's syndrome but involving mainly the axial skeleton. The synthetic glucocorticoid prescription drug prednisone is a main candidate after prolonged intake. Bisphosphonates are beneficial in reducing the risk of vertebral fractures. Some professional guidelines recommend prophylactic calcium and vitamin D supplementation in patients who take the equivalent of more than 30 mg hydrocortisone, especially when this is in excess of three months. The use of thiazide diuretics, and gonadal hormone replacement has also been recommended, with the use of calcitonin, bisphosphonates, sodium fluoride or anabolic steroids also suggested in refractory cases. Alternate day use may not prevent this complication.

<span class="mw-page-title-main">Cone beam computed tomography</span> Medical imaging technique

Cone beam computed tomography is a medical imaging technique consisting of X-ray computed tomography where the X-rays are divergent, forming a cone.

FRAX is a diagnostic tool used to evaluate the 10-year probability of bone fracture risk. It was developed by the University of Sheffield. FRAX integrates clinical risk factors and bone mineral density at the femoral neck to calculate the 10-year probability of hip fracture and the 10-year probability of a major osteoporotic fracture. The models used to develop the FRAX diagnostic tool were derived from studying patient populations in North America, Europe, Latin America, Asia and Australia.

The trabecular bone score is a measure of bone texture correlated with bone microarchitecture and a marker for the risk of osteoporosis. Introduced in 2008, its main projected use is alongside measures of bone density in better predicting fracture risk in people with metabolic bone problems.

Dual X-ray absorptiometry and laser technique (DXL) in the area of bone density studies for osteoporosis assessment is an improvement to the DXA Technique, adding an exact laser measurement of the thickness of the region scanned. The addition of object thickness adds a third input to the two x-ray energies used by DXA, better solving the equation for bone and excluding more efficiently these soft tissues components.

Ronald Marc Summers is an American radiologist and senior investigator at the Diagnostic Radiology Department at the NIH Clinical Center in Bethesda, Maryland. He is chief of the Clinical Image Processing Service and directs the Imaging Biomarkers and Computer-Aided Diagnosis (CAD) Laboratory. A researcher in the field of radiology and computer-aided diagnosis, he has co-authored over 500 journal articles and conference proceedings papers and is a coinventor on 12 patents. His lab has conducted research applying artificial intelligence and deep learning to radiology.

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<span class="mw-page-title-main">Radiofrequency Echographic Multi Spectrometry</span> Medical diagnostic

Radiofrequency Echographic Multi Spectrometry (REMS) is a non-ionizing technology for osteoporosis diagnosis and for fracture risk assessment. REMS processes the raw, unfiltered ultrasound signals acquired during an echographic scan of the axial sites, femur and spine. The analysis is performed in the frequency domain. Bone mineral density (BMD) is estimated by comparing the results against reference models.

Mary Larsen Bouxsein is an American biomechanical engineer and an orthopedic researcher. She is the president of the American Society for Bone and Mineral Research, director of the Centre of Advanced Orthopaedic Studies at Beth Israel Deaconess Medical Center, professor at the department of Orthopedic Surgery at Harvard Medical School. She is known for her work on bone density and the use of imaging to define the factors leading to bone fractures.

References

  1. 1 2 3 J. E. Adams, "Quantitative computed tomography.," European journal of radiology, vol. 71, no. 3, pp. 415-24, Sep. 2009.
  2. ↑ K G Faulkner, C C Glüer, S Grampp, and H K Genant, "Cross-calibration of liquid and solid QCT calibration standards: Corrections to the UCSF normative data" Osteoporosis International, vol. 3, no. 1, pp. 36-42, 1993.
  3. ↑ Reflections on Development of Quantitative Computed Tomography
  4. ↑ K. Engelke, A. Mastmeyer, V. Bousson, T. Fuerst, J.-D. Laredo, and W. a Kalender, "Reanalysis precision of 3D quantitative computed tomography (QCT) of the spine.," Bone, vol. 44, no. 4, pp. 566-72, Apr. 2009.
  5. 1 2 B. C. C. Khoo et al., "Comparison of QCT-derived and DXA-derived areal bone mineral density and T scores.," Osteoporosis International, vol. 20, no.9, pp. 1539-45, Sep. 2009.
  6. ↑ J. S. Bauer, S. Virmani, and D. K. Mueller, "Quantitative CT to assess bone mineral density as a diagnostic tool for osteoporosis and related fractures," MedicaMundi, vol. 54, no. 2, pp. 31-37, 2010.
  7. ↑ R. M. Summers et al., "Feasibility of simultaneous computed tomographic colonography and fully automated bone mineral densitometry in a single examination.," Journal of computer assisted tomography, vol. 35, no. 2, pp. 212-6, 2011.
  8. ↑ A. El Maghraoui and C. Roux, "DXA scanning in clinical practice.," QJM, vol. 101, no. 8, pp. 605 17, Aug. 2008.
  9. ↑ E. W. Yu, B. J. Thomas, J. K. Brown, and J. S. Finkelstein, "Simulated increases in body fat and errors in bone mineral density measurements by DXA and QCT.," Journal of Bone and Mineral Research, vol. 27, no. 1, pp. 119-124, Sep. 2011.
  10. ↑ G. Guglielmi et al., "Effect of Spinal Degenerative Changes on Volumetric Bone Mineral Density of the Central Skeleton as Measured by Quantitative Computed Tomography," Acta Radiologica, vol. 46, no. 3, pp. 269-275, Jan. 2005.
  11. ↑ J. A. Smith, J. A. Vento, R. P. Spencer, and B. E. Tendler, "Aortic Calcification Contributing to Bone Densitometry Measurement," Journal of Clinical Densitometry, vol. 2, no. 2, pp. 181-183, Jun. 1999.
  12. ↑ G. Liu, M. Peacock, O. Eilam, G. Dorulla, E. Braunstein, and C. C. Johnston, "Effect of osteoarthritis in the lumbar spine and hip on bone mineral density and diagnosis of osteoporosis in elderly men and women.," Osteoporosis International, vol. 7, no. 6, pp. 564-9, Jan. 1997.
  13. ↑ J. C. Prior et al., "Premenopausal ovariectomy-related bone loss: a randomized, double-blind, one-year trial of conjugated estrogen or medroxyprogesterone acetate.," Journal of Bone and Mineral Research, vol. 12, no. 11, pp. 1851-63, Nov. 1997.
  14. ↑ Pickhardt et al., "Simultaneous screening for osteoporosis at CT colonography: Bone mineral density assessment using MDCT attenuation techniques compared with the DXA reference standard," Journal of Bone and Mineral Research, vol. 26, no. 9, pp. 2194-203, 2011, ,
  15. ↑ Bauer et al., "Volumetric quantitative CT of the spine and hip derived from contrast-enhanced MDCT: conversion factors," American Journal of Roentgenology, vol. 188, no.5, pp. 1294-301, 2007.
  16. ↑ American College of Radiology, "ACR Practice Guideline for the Performance of Quantitative Computed Tomography (QCT) Bone Densitometry," 2008.
  17. ↑ E. M. Lewiecki et al., "International Society for Clinical Densitometry 2007 Adult and Pediatric Official Positions.," Bone, vol. 43, no. 6, pp. 1115-21, Dec. 2008.
  18. ↑ E. M. Lewiecki et al., "Official Positions for FRAX Bone Mineral Density and FRAX simplification from Joint Official Positions Development Conference of the International Society for Clinical Densitometry and International Osteoporosis Foundation on FRAX.," Journal of Clinical Densitometry, vol. 14, no. 3, pp. 226-36.
  19. ↑ Müller A, Rüegsegger E, Rüegsegger P (1989). "Peripheral QCT: a low-risk procedure to identify women predisposed to osteoporosis". Phys Med Biol. 34 (6): 741–9. Bibcode:1989PMB....34..741M. doi:10.1088/0031-9155/34/6/009. PMID   2740441. S2CID   250737312.
  20. ↑ Goolsby, Marci A.; Boniquit, Nicole (2016-11-30). "Bone Health in Athletes". Sports Health. 9 (2): 108–117. doi:10.1177/1941738116677732. ISSN   1941-7381. PMC   5349390 . PMID   27821574.
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