Spring 2022

Volume 32 – Spring 2022

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Üstün Aydıngöz, MD

Hacettepe University School of Medicine

Ankara, Turkey

Adalet Elçin Yıldız, MD

Hacettepe University School of Medicine

Ankara, Turkey

IN PRACTICE

The best of two worlds: CT-like imaging of mineralized structures with MR

Üstün Aydıngöz, MD

Hacettepe University School of Medicine

Ankara, Turkey

Adalet Elçin Yıldız, MD

Hacettepe University School of Medicine

Ankara, Turkey

oZTEo is a new application that provides enhanced cortical bone versus softtissue contrast in MR. This novel technique, based on the radial k-space filling ZTE acquisition, carries the promise of allowing facilities to be a one-stop shop for bone imaging and adds considerable clinical value to orthopedic exams. oZTEo is inherently motion-insensitive and complements a conventional soft-tissue MR exam by delivering 3D isotropic imaging with the familiar bright bone contrast of CT.

In Turkey, a G20 country with a midsized population, universal healthcare covers its 84 million citizens. According to the OECD and the Turkish Health Ministry data, the use of MR in the country is the most extensive in the world, with 195 MR exams per 100,000 citizens in 2019.^1,2 Located in the capital city of Ankara, Hacettepe University Medical Center is comprised of three hospitals serving many patients from all over Turkey.

Üstün Aydıngöz, MD, has been a dedicated musculoskeletal (MSK) radiologist at Hacettepe for the last 26 years and a Professor of Radiology at Hacettepe University School of Medicine since 2003. Adalet Elçin Yıldız, MD, is also an MSK radiologist at Hacettepe, as well as a board-certified pediatric radiologist and an Associate Professor of Radiology. The work of Drs. Aydıngöz and Yıldız, along with their now-retired colleague Fatma Bilge Ergen, MD, on zero echo-time (ZTE) imaging received a Magna Cum Laude Award at the 107th Scientific Assembly and Annual Meeting of the Radiological Society of North America (RSNA) in 2021. Titled “Radiographs and CT within Musculoskeletal MRI: Challenges, Optimization and Applications of Zero Echo-Time Imaging,” their exhibit outlined how the facility is optimizing the oZTEo sequence, as well as employing it in a vast array of MSK conditions.³ The team’s work will be featured in a forthcoming article in RadioGraphics, the major educational publication of the RSNA.

Hacettepe University Medical Center attracts a diverse patient population, including many with MSK conditions (developmental, inflammatory, infectious, traumatic, metabolic and oncologic) in children and adults, which made it possible to implement oZTEo on a wide variety of abnormalities. Drs. Aydıngöz and Yıldız say MSK MR with oZTEo offers the best of both worlds: It combines exquisite depiction of MSK soft tissues and bone marrow on MR with the radiograph- or CT-like rendering of mineralized structures.

Implementing oZTEo at Hacettepe

After installing a SIGNA™ Architect 3.0T system at Hacettepe, the radiology department optimized the oZTEo imaging sequence with technical guidance and assistance from GE Healthcare. Over the past year, the doctors have incorporated the oZTEo sequence into their routine MSK MR protocols and they continue to assess the feasibility of adding this three- to four-minute sequence to existing standard protocols.

They believe all applications with a focus on tissues and structures with very short T2s (<1 ms), typically considered MR invisible, can potentially benefit from using oZTEo. As an added feature to the already superior soft-tissue detail MR provides, the oZTEo sequence enables efficient and ionizing radiation- free evaluation of calcified structures.

The very short T2 of cortical bone, due mostly to its low water content, results in a signal void appearance on conventional MR sequences. Rapid radiofrequency switching in the oZTEo sequence from transmit to receive mode enables the acquisition of the quickly decaying signal, thereby capturing whatever little signal is present in cortical bone.

In oZTEo, readout gradients are switched on before the RF pulse to ensure immediate data acquisition after excitation. As gradient encoding starts instantaneously upon signal acquisition, TE is almost zero. Gradients are on continuously – rather than being rapidly turned on and off – and modulated between repetitions to sample data along radial trajectories in k-space. The absence of on and off gradient switching results in a virtually silent scan during oZTEo.

Post-processing oZTEo produces CT-like images that mostly distinguish and delineate cortical bone from other surrounding short T2 structures, such as the joint capsule, labra and menisci. Near-isotropic acquisition is possible with oZTEo, allowing multiplanar and radial reformatting as with CT, and minimum intensity projection to produce radiograph-like images.

The most promising areas regarding the use of oZTEo in MSK MR include morphometric analyses, such as the measurement of critical shoulder angle (CSA) (Figure 1), alpha angle for femoral impingement and the assessment of the lower extremity rotation profile. It is also promising in glenoid bone stock and glenoid track estimation, depiction of subtle fractures, osseous loose bodies, calcific deposits in soft tissues, bone erosions (Figure 2) in inflammatory arthritis, and assessment of new bone formation and cortical violation (Figure 3) in bone tumors. What’s more, the virtually silent acquisition during oZTEo is especially appealing in MR examinations performed in pediatrics, particularly very young children.

Figure 1. A 47-year-old woman with shoulder pain. (A) CSA is an indicator of both glenoid inclination and acromial coverage of the humeral head and is conventionally measured on an optimally positioned true anteroposterior shoulder radiograph (Grashey view). The CSA forms between two lines, one connecting the superior and inferior edges of the glenoid cavity and the other between the inferior edge of the glenoid cavity and lateral edge of the acromion. (B) The CSA measurement was exactly reproduced on a 25 mm thick slab reformatted coronal oblique minimum intensity projection ZTE MR image of the same patient.

Figure 2. A 39-year-old woman with ankylosing spondylitis. (A) Coronal oblique reformatted oZTEo image shows bilateral sacroiliac erosions (arrowheads) with CT-like clarity and better than the (B) contemporaneous Ferguson radiograph. (C) Coronal oblique T2w FatSat image displays the right-sided periarticular osteitis denoting active sacroiliitis, yet fails to show the erosions as exquisitely as oZTEo.

Figure 3. Atypical cartilaginous tumor in a 52-year-old man with left hip pain. (A) Coronal T2w FatSat MR image shows a cartilaginous matrix tumor in the medial aspect of the left femoral neck, which displays cortical breakthrough (arrow) on the corresponding (B) coronal reformatted oZTEo MR and (C) CT images (CT is a reformat from a PET/CT with a 2.5 mm-thick source image set). Cortex violation is an indicator of low-grade chondrosarcoma, which is a difficult diagnosis to make on histological ground alone and necessitates close radiology–pathology collaboration.

“We now routinely use oZTEo to image osseous Bankart lesions. The oZTEo images are comparable to CT, allowing us to avoid an additional exam. This is poised to become a favorite application of oZTEo for our orthopedic surgeon colleagues.”

Dr. Üstün Aydıngöz

For shoulder problems, the CSA is considered by many as an important prognosticator for rotator cuff tears or shoulder osteoarthritis and is traditionally measured on Grashey view radiographs. A soonto- be-published study by Drs. Yıldız and Aydıngöz showed that the CSA measurement on oZTEo MR with anatomical point cross-referencing was significantly different than on Grashey view radiography, even with optimal radiographic positioning performed.⁴ According to this study, more stringent measures are needed to ensure the reliability of Grashey view radiographs, even when other standards of sufficient X-ray exposure are met. Actually, some studies that did not support the CSA to be a prognosticator for shoulder disease might have suffered from inaccurate measurement of the CSA, due to positional variability associated with the Grashey view.

In fact, oZTEo can be used for measuring the CSA in patients with shoulder problems (Figure 1), many of whom will be undergoing shoulder MR in any case.

Optimizing oZTEo

To achieve flat soft-tissue contrast (i.e., minimize T1 contrast), a low flip angle (1° or 2°) PD-weighted sequence is used in oZTEo. To avoid chemical shift interferences at fat-water interfaces, a pixel bandwidth larger than the fat-water chemical shift (i.e., 3.5 ppm) needs to be employed. This is usually why oZTEo is better achieved at 3.0T with sequence bandwidths of 60-90 kHz.

“As with many things in life, optimization of oZTEo is a matter of trade-offs,” says Dr. Aydıngöz. Increasing the receiver bandwidth reduces the chemical shift artifact, but too much of it also blurs the images. Increasing the flip angle increases the signal-to-noise ratio (SNR), but also increases the T1 contrast in the background tissue. Increasing the field of view increases the SNR but also “softens” the images.

Coil selection is another consideration in the optimization of some applications of oZTEo. For conventional sagittal lumbar spine MR sequences, an anterior saturation band is used to eliminate bowel motion artifacts and only a posterior array (PA) coil is used. For lumbar spine oZTEo imaging, however, the combination of a PA coil and an anterior array (AA) coil is recommended for better SNR.

Overcoming the challenges of oZTEo

Intraarticular gas and susceptibility artifacts from surgical materials comprise the main pitfalls of oZTEo by mimicking ossification or calcification. Likewise, micrometallic debris artifacts, hemosiderin deposition and the content of some ganglion cysts appear white on the oZTEo images. Correlation with standard MR sequences (Figure 4), however, is mostly sufficient to overcome these pitfalls, according to Drs. Aydıngöz and Yıldız.

Figure 4. A 49-year-old woman with medial compartment knee osteoarthritis. What appears to be an intraarticular loose body (arrow) on the (A) sagittal oZTEo image actually represents gas within the degenerated joint space, which is easily ascertained by correlation with the corresponding (B) T2w FatSat and (C) gradient echo images. Note the blooming artifact associated with gas on (C).

While the current spatial resolution of oZTEo MR is not as high as that of CT or radiography, the near-isotropic characteristics of the 3D oZTEo sequence allows easy reformatting in any plane on PACS and volume rendering on a workstation, affording sets of images for careful scrutiny of suspicious features and cross-referencing them on the conventional MR sequences.

Opportunities for oZTEo

Dr. Aydıngöz and Dr. Yıldız conclude there are several opportunities with oZTEo:

oZTEo provides radiograph- and CT-like bone images from MR
oZTEo can be used complementarily along with – and at times instead of – radiographs and/or CT in many traumatic, inflammatory/rheumatological and oncologic musculoskeletal conditions
Morphometric measurements, such as the critical shoulder angle and the alpha angle, can be reliably made with oZTEo, obviating the need for radiographs or CT
The virtually silent scan is an extra benefit of oZTEo for patient comfort.