How to Plan an Ankle MRI Protocol with Metal Implant

Intro to Ankle MRIs with Metal Implants

How to Perform an Ankle MRI with Metal Implant

This hardware setup is widely used in clinical practice. It balances acquisition time, image quality, and patient comfort.

4. Capture the Initial Localizer Images

Before we can perform any MRI protocol, we must always capture initial localizer images of the patient. These images act as a guide for planning the detailed scans we will perform next.

We should always capture localizers in three planes:

1. Axial
2. Sagittal
3. Coronal

Once acquired, upload the initial localizer images into the three viewports.

Then, scroll through each of the image stacks to locate a central slice that clearly shows the anatomy of the ankle.

✅ Correct Setup of Localizer Images for Ankle MRI:

Part 2: Plan and Acquire the Protocol Sequences

When all preparations are ready, we can start planning and acquiring the protocol sequences.

Let's go through the pulse sequences a standard ankle MRI protocol with metal implants includes, why we perform them, and how to set them up.

The 5 Sequences of a Standard Ankle Protocol with Metal Implants

1. Sagittal T1 TSE
2. Sagittal PD STIR
3. Coronal PD STIR
4. Axial PD STIR
5. Axial T2 TSE

We mainly use Turbo Spin Echo sequences with STIR fat suppression for this study. These sequences handle metal artifacts much better than gradient echo sequences and provide consistent fat suppression even near metal implants.

STIR fat suppression also works better than spectral suppression around metal, because it doesn't rely on a uniform magnetic field.

In the sections below, we go through how to plan and set up each sequence.

1. Sagittal T1 TSE

✅ Correct Planning:

Planning Instructions:

• Use the medial and lateral malleoli as your anatomical references.
• Align the slices as follows:
  • Axial localizer: Angle slices perpendicular to the malleoli and center over the ankle joint.
  • Sagittal localizer: Center the slice package over the ankle.
  • Coronal localizer: Ensure slices are parallel to the tibial bone.
• Use appropriate geometry parameters:
  • Slice number: Enough slices to cover the ankle from medial to lateral (20–25 slices).
  • Slice thickness: 3 mm for high resolution to visualize tissues around the metal.
  • Slice gap: 0.3 mm (10% of slice thickness) to maintain continuity.
• Set the fold-over direction (phase encoding) to anterior–posterior (AP) to steer wraparound artifacts away from the implant.

Parameters for Sagittal T1 TSE:

2. Sagittal PD STIR TSE

✅ Correct Planning:

Planning Instructions:

• Copy the slice geometry and planning from the previous Sagittal T1 TSE sequence.
• Keep the same slice angulation, coverage, and positioning to ensure images of different contrasts can be clearly compared.

Parameters for Sagittal PD STIR:

3. Coronal PD STIR TSE

✅ Correct Planning:

Planning Instructions:

• Use the lateral and medial malleoli as your anatomical references.
• Align the slices as follows:
  • Axial localizer: Ensure slices are parallel to the malleoli.
  • Sagittal localizer: Angle slices parallel to the tibial bone and center the slice package and angle the slices parallel to the tibial bone.
  • Coronal localizer: Center the slice package.
• Use appropriate geometry parameters:
  • Slice number: Cover the ankle from anterior to posterior (25–30 slices).
  • Slice thickness: 3 mm for consistent resolution.
  • Slice gap: 0.3 mm to maintain continuity.
• Set the fold-over direction (phase encoding) to right–left (RL) since no anatomy extends outside FOV in this direction.

Parameters for Coronal PD STIR:

4. Axial PD STIR TSE

✅ Correct Planning:

Planning Instructions:

• Use the tibiotalar joint as your anatomical reference (if visible despite metal artifacts). If it’s hidden by artifacts, use higher spatial resolution to overcome the metal artifacts.
• Align the slices as follows:
  • Axial localizer: Center the slice package.
  • Sagittal localizer: Angle slices perpendicular to the tibial shaft.
  • Coronal localizer: Ensure slices cover the joint space evenly.
• Use appropriate geometry parameters:
  • Slice number: Enough to cover the ankle from top to bottom (30–35 slices).
  • Slice thickness: 3 mm for high resolution.
  • Slice gap: 0.3 mm for continuity.
• Set the fold-over direction (phase encoding) to right–left (RL) to avoid wraparound.

Parameters for Axial PD STIR:

5. Axial T2 TSE

✅ Correct Planning:

Planning Instructions:

• Copy the slice geometry and planning from the previous Axial PD STIR sequence.
• Keep the same slice angulation, coverage, and positioning to ensure images of different contrasts can be clearly compared.

Parameters for Axial T2 TSE:

How to Avoid Artifacts When Planning the Sequences

The table below lists the 5 common ankle artifacts when imaging with metal implants, and what techniques you can use to avoid them:

Part 3: Review the Images

Finally, we will review the images to ensure all the anatomical information we need is clear.

These key structures must be clearly visible in an ankle MRI with metal implants:

1. Bone-implant interface
2. Surrounding bone marrow
3. Ligaments and tendons (if not obscured by metal)
4. Joint spaces and cartilage
5. Soft tissues around the implant
6. Signs of fluid collection or inflammation

Below, we will go through all the different image contrasts and explain their specific role in imaging the ankle with metal implants.

T1 TSE – Highlight Bone and Structural Changes

T1-weighted imaging makes fat appear bright and fluid dark. This contrast is ideal for fat-rich tissues and structural abnormalities. T1 shows anatomical structures clearly, since it helps us see where different solid tissues like muscle and fat meet.

In ankle MRI with metal implants, T1 sequences are key for evaluating the bone-implant interface, detecting loosening, and assessing for avascular necrosis. The bright fat signal helps identify bone marrow changes and stress fractures around the implant.

We capture the sagittal view to assess the ankle from a lateral perspective. This helps evaluate the tibiotalar joint and surrounding structures along the length of the ankle.

✅ Sagittal T1 TSE – Correct Image Example:

‍

Things to Look for in Sagittal T1:

• Clear visualization of bone-implant interface
• Normal bright marrow signal away from metal
• Dark fluid collections suggesting infection
• Structural integrity of surrounding bone

PD STIR – Clearest View of Inflammation and Fluid

STIR (Short Tau Inversion Recovery) suppresses fat signals completely, which makes water-rich tissues stand out even clearer than on T2. This makes STIR ideal for detecting subtle fluid-related issues, such as edema, inflammation, and infections where increased water content would otherwise be obscured by fat.

In ankle MRI with metal implants, STIR is crucial for identifying infections, bone marrow edema, and soft tissue inflammation. Unlike spectral fat suppression, STIR works reliably near metal because it doesn't depend on a uniform magnetic field.

We capture sagittal, coronal, and axial views to thoroughly evaluate all aspects of the ankle. Each plane provides unique information about different anatomical structures and pathology.

✅ Sagittal PD STIR – Correct Image Example:

Things to Look for in Sagittal PD STIR:

• Bright signal indicating marrow edema or infection
• Fluid collections around the implant
• Soft tissue inflammation or abscess formation
• Integrity of tendons and ligaments

✅ Coronal PD STIR – Correct Image Example:

Things to Look for in Coronal PD STIR:

• Symmetric assessment of both malleoli
• Fluid in joint spaces
• Bone marrow edema patterns
• Soft tissue swelling or inflammation

✅ Axial PD STIR – Correct Image Example:

Things to Look for in Axial PD STIR:

• Cross-sectional view of implant position
• Circumferential fluid collections
• Soft tissue inflammation patterns
• Tendon and ligament abnormalities

T2 TSE – Highlight Fluid and Soft Tissue Detail

T2-weighted imaging makes fluids appear bright. This contrast is ideal for tissues and abnormalities with high water content.

In ankle MRI with metal implants, T2 sequences help assess joint effusions, tendon tears, and ligament injuries. The bright fluid signal makes it easy to identify abnormal fluid collections that might indicate complications.

✅ Axial T2 TSE – Correct Image Example:

Things to Look for in Axial T2:

• Joint effusions appearing bright
• Tendon integrity and signal
• Ligament tears or thickening
• Cystic changes or fluid collections

Final Checks:

Before finishing an ankle MRI with metal implants, always check these 5 points to ensure diagnostic quality:

1. Metal Artifact Management: Ensure metal artifacts are minimized enough to visualize critical structures around the implant.
2. Bone-Implant Interface: The interface must be visible to assess for loosening or osteolysis.
3. Fluid Detection: STIR sequences must clearly show any abnormal fluid collections or marrow edema.
4. Soft Tissue Coverage: All sequences must adequately cover tendons, ligaments, and muscles around the ankle.
5. Image Quality: Images must have adequate SNR despite high bandwidth settings, with minimal motion or wraparound artifacts.

Bonus: Why Turbo Spin Echo with STIR Fat Suppression Is Best for Metal Implants

When scanning ankles with metal implants, not all sequence types handle artifacts equally well.

To understand why turbo spin echo (TSE) with STIR fat suppression is preferred, it helps to compare it directly to two common alternatives.

Test 1 – Turbo Spin Echo vs Gradient Echo

If you keep the same scan parameters (TR, TE, matrix, bandwidth, and geometry) and simply change the sequence type from turbo spin echo to gradient echo, the image quality changes significantly.

Turbo spin echo will typically show the ankle anatomy clearly, even close to the implant. The metal artifact is still present, but the structures around it are usually well-defined.

With gradient echo, however, the image near the metal often becomes unreadable. The artifact appears larger and more intense, and nearby anatomy may be completely obscured.

This happens because turbo spin echo includes refocusing pulses that correct many of the distortions caused by magnetic field variations around metal.

Gradient echo, however, lacks these pulses, so the artifacts appear stronger and cover more of the image.

Test 2 – STIR vs Spectral Fat Suppression

Now consider a sagittal PD sequence where all parameters stay the same, but you switch from STIR to spectral fat suppression.

With STIR, fat suppression remains even across the field of view. Bone marrow shows a uniform low signal, and subcutaneous fat appears consistently dark.

But with spectral fat suppression, the fat suppression may appear patchy. You might see bright signals remaining in bone marrow and subcutaneous fat, especially near the implant.

This difference is due to how the sequences work.

Spectral fat suppression works by identifying fat based on its frequency. It requires a uniform magnetic field to apply the suppression accurately. But metal implants disrupt that uniformity, which causes the technique to fail in some regions.

STIR works differently. It uses timing (inversion recovery) to suppress fat, not frequency. This allows STIR to perform more reliably in areas with magnetic field distortion, such as around metal implants.

This is why ankle MRI protocols with implants typically use TSE pulse sequences with STIR for fat suppression.

• Turbo spin echo handles metal distortion better than gradient echo.
• STIR fat suppression performs more reliably near metal than spectral fat suppression.

This combination helps reduce artifacts, improves visualization of soft tissues, and enables better diagnostic quality around implants.

‍