What Is QCT? Bone Density Measurement and the Difference Between QCT and DXA

Created: 06.11.2024 · Last Updated: 11.08.2026 · Category: Radiology · Prepared by the Academic Hospital Web and Editorial Board.

Quantitative Computed Tomography (QCT) is an imaging technique that uses computed tomography data to measure bone mineral density volumetrically.

QCT can be used at the spine and hip and can provide three-dimensional information about selected regions of bone.

However, QCT is not the only method used to assess bone mineral density. Central DXA remains the most widely used and best-standardised technique for osteoporosis diagnosis, screening and treatment decisions.

QCT and DXA are not interchangeable tests. The appropriate method depends on fracture risk, age, spinal anatomy, previous imaging and the clinical question being addressed.

What Is Quantitative Computed Tomography (QCT)?

QCT uses CT imaging to provide quantitative measurements of bone mineral density in selected skeletal regions.

Unlike conventional DXA, which measures bone mineral density across a two-dimensional projected area, QCT can calculate volumetric bone mineral density.

In spinal QCT, trabecular bone can be assessed separately. Trabecular bone is metabolically active and may provide useful additional information in selected patients.

QCT may be used:

  • To assess low bone mineral density
  • As part of osteoporosis and fracture-risk assessment
  • When structural changes may interfere with interpretation of spinal DXA
  • For monitoring bone density in selected clinical situations

How Does QCT Work?

During QCT, CT images of the relevant anatomical region are obtained and analysed using dedicated bone-density software.

The software evaluates defined regions of bone against calibration or reference data and calculates volumetric bone mineral density (vBMD).

The process generally includes:

  1. Positioning the patient on the CT table
  2. Acquiring images of the relevant spine or hip region
  3. Analysing the images with validated software
  4. Reporting quantitative bone-density measurements

What Does QCT Measure?

A key feature of QCT is its ability to measure bone mineral density within a three-dimensional volume.

Spinal QCT can specifically assess trabecular bone mineral density. This may provide useful information because trabecular bone can respond relatively quickly to metabolic changes.

However, QCT and DXA measure bone density differently. Results from different techniques and devices should not be directly compared unless an appropriate validated method is available.

Who May Have QCT?

There is no general recommendation that everyone above a particular age should undergo QCT.

QCT may be considered in selected situations such as:

  • When spinal DXA is difficult to interpret: Advanced degenerative changes, osteoarthritis, scoliosis or other structural abnormalities may artificially increase spinal DXA measurements.
  • When central DXA cannot be performed or does not provide sufficient information
  • When volumetric bone mineral density is clinically useful
  • When suitable existing CT images can be used for validated opportunistic bone-density assessment

The need for QCT should be decided according to the individual's clinical circumstances.

What Is DXA (DEXA)?

Dual-Energy X-ray Absorptiometry (DXA) uses two X-ray energy levels to measure bone mineral density with a very low radiation dose.

Although the term DEXA is widely used, DXA is the standard abbreviation.

Central DXA most commonly evaluates:

  • Lumbar spine
  • Total hip
  • Femoral neck
  • The forearm in selected situations

Because DXA is quick, non-invasive, widely available and highly standardised, it remains the principal method used for osteoporosis assessment.

What Is the Difference Between QCT and DXA?

Feature QCT DXA
Measurement Volumetric BMD Areal BMD
Imaging CT-based, three-dimensional Two-dimensional projection
Separate trabecular assessment Possible Not directly separated
Standardisation for osteoporosis diagnosis Depends on technique and anatomical site Established international standard
Radiation CT-based and protocol dependent; generally higher than DXA Very low dose
Spinal degenerative changes Selected trabecular analysis may reduce some confounding effects Degenerative change can artificially increase spinal BMD
Three-dimensional imaging does not automatically make QCT “more accurate” than DXA. The techniques measure different parameters. Central DXA remains the preferred method for most osteoporosis diagnosis and treatment decisions.

QCT or DXA: Which Is Preferred?

The International Society for Clinical Densitometry recommends central DXA measurements of the spine and hip as the preferred method for making therapeutic decisions whenever possible.

QCT may be an important alternative or complementary technique:

  • When DXA cannot be performed
  • When structural changes make DXA difficult to interpret reliably
  • When volumetric BMD information is clinically useful
  • When validated opportunistic assessment can be performed on suitable existing CT images

The clinically relevant question is therefore not simply whether QCT or DXA is “better”, but which test is most appropriate for the individual patient.

Who Should Have Bone Density Testing?

Bone-density testing should be based on fracture risk and clinical circumstances rather than an arbitrary age threshold for QCT.

ISCD indications for BMD testing include:

  • Women aged 65 and older
  • Postmenopausal women younger than 65 with risk factors for low bone mass
  • Women during the menopausal transition who have clinical fracture-risk factors
  • Men aged 70 and older
  • Men younger than 70 with risk factors for low bone mass
  • Adults with a fragility fracture
  • Adults with diseases or conditions associated with bone loss
  • Adults taking medications associated with bone loss
  • People being considered for or monitored during osteoporosis treatment

The 2025 U.S. Preventive Services Task Force recommendation also supports osteoporosis screening with DXA in women aged 65 and older and in younger postmenopausal women who are found to be at increased fracture risk.

How Are QCT and DXA Results Interpreted?

DXA Results

DXA reports commonly include a T-score and a Z-score.

The T-score compares bone mineral density with a healthy young-adult reference population.

For postmenopausal women and men aged 50 and older, the general DXA densitometric classification includes:

  • T-score of -1.0 or above: Normal bone density
  • T-score between -1.0 and -2.5: Low bone mass
  • T-score of -2.5 or below: Densitometric osteoporosis

The Z-score compares bone mineral density with a reference population of a similar age and is particularly relevant in premenopausal women, men younger than 50 and children.

QCT Results

Spinal QCT commonly reports volumetric bone mineral density, for example in mg/cm³.

QCT measurements should not automatically be interpreted using the same T-score framework as standard DXA.

According to the ISCD, femoral-neck and total-hip T-scores calculated from validated two-dimensional projections of QCT data can be equivalent to corresponding DXA T-scores for diagnosis under defined conditions.

Bone density is only one component of fracture-risk assessment. Age, previous fractures, family history, medication use, menopausal status and other clinical risk factors should also be considered.

Does QCT Use Radiation?

Yes. QCT is based on computed tomography and therefore uses ionising radiation.

The dose depends on factors including:

  • Scanner model
  • Anatomical region
  • Scan length
  • Acquisition parameters such as kV and mAs
  • Whether a dedicated low-dose protocol is used

It is therefore not appropriate to state that every QCT examination uses a fixed number of slices or a single fixed radiation dose.

DXA uses substantially less radiation than QCT. When QCT and DXA provide comparable clinical information, ISCD recommends DXA in order to limit radiation exposure.

If suitable CT images have already been obtained for another medical reason, validated opportunistic QCT techniques may allow bone-density assessment without acquiring a separate CT scan specifically for BMD.

Tell your doctor and radiology team if you are or may be pregnant before undergoing QCT or another X-ray/CT examination. The clinical need for imaging and possible alternatives should be assessed individually.

Is Preparation Required Before QCT?

Preparation depends on the specific imaging protocol.

Patients may be asked to:

  • Remove metal accessories, belts or clothing over the area being scanned
  • Inform the imaging team if pregnancy is possible
  • Bring previous bone-density examinations or relevant imaging studies when available

The imaging centre can provide any protocol-specific instructions before the appointment.

Assess Bone Density With the Appropriate Method

The choice between DXA and QCT can be based on age, fracture risk, spinal anatomy, previous imaging and the individual clinical question.

Frequently Asked Questions

What is QCT?
QCT is an imaging technique that uses computed tomography data to measure bone mineral density volumetrically. It can be used particularly at the spine and hip.
Is QCT more accurate than DXA?
It is not correct to say that one method is more accurate in every situation. QCT provides volumetric and three-dimensional information, while DXA is more widely used and more strongly standardised for osteoporosis diagnosis, screening and treatment decisions. The appropriate method depends on the patient's clinical circumstances.
Who may have QCT?
There is no general age-based rule for QCT. It may be considered when DXA cannot be performed or is difficult to interpret because of advanced degenerative spinal changes, when volumetric bone mineral density is clinically useful or when suitable existing CT images can be used for additional assessment.
How long does a QCT scan take?
The duration depends on the scanner and protocol. Image acquisition is generally brief, although the total appointment time including preparation, positioning and analysis can vary between imaging centres.
Does QCT use radiation?
Yes. QCT is CT-based and therefore uses ionising radiation. The dose depends on the scanner, anatomical region and imaging protocol and is generally higher than DXA. The examination should therefore be selected according to clinical need.
Does QCT use a T-score?
Spinal QCT is commonly reported as volumetric bone mineral density and should not automatically be interpreted using standard DXA T-scores. In certain validated QCT hip applications, femoral-neck and total-hip T-scores derived from two-dimensional projections may be equivalent to DXA values.
Why is DXA more commonly used for osteoporosis?
DXA is the principal bone-density technique because it uses a very low radiation dose, is widely available, is strongly standardised and has extensive reference data for osteoporosis diagnosis and treatment monitoring.
Academic Hospital note: QCT and DXA have different strengths in bone-density assessment. The appropriate method should be selected according to fracture risk, spinal and hip anatomy, previous measurements and the clinical indication rather than age alone. You can book an appointment.

References

  1. Academic Hospital. QCT (Kemik Yoğunluğu Ölçümü) Nedir? Kimlere Yapılmaktadır?
  2. International Society for Clinical Densitometry. 2023 ISCD Official Adult Positions
  3. American College of Radiology. ACR Appropriateness Criteria: Osteoporosis and Bone Mineral Density
  4. Radiological Society of North America / American College of Radiology. Osteoporosis and Bone Mineral Density
  5. Radiological Society of North America / American College of Radiology. Bone Density Scan (DEXA or DXA)
  6. U.S. Preventive Services Task Force. Osteoporosis to Prevent Fractures: Screening