THE ROLE OF ZERO ECHO TIME (ZTE) AND VOLUMETRIC INTERPOLATED BREATH-HOLD EXAMINATION (VIBE) MRI SEQUENCES

– Written by Maria Lua Sampaio Gulde, Marcelo Bordalo, Qatar

INTRODUCTION   

Bone stress injury (BSI) accounts for approximately 10% to 20% of all sports-related injuries, particularly in athletes or individuals exposed to repetitive mechanical loading. These injuries occur when abnormal and repetitive stress is applied to normal bone, resulting in cumulative microdamage¹. Over time, if the mechanical load exceeds the bone’s capacity to repair, this process can progress to complete fractures. BSIs must be differentiated from insufficiency fractures, which are caused by normal physiologic stress acting on structurally weakened bone, such as in elderly patients with osteoporosis or individuals with metabolic disorders².

The distribution of bone stress injuries varies with the sport and the specific mechanical forces involved. However, they are more common in the lower extremities, accounting for up to 75% of exertional lower leg pain². The most frequently affected sites include the tibia, calcaneus, metatarsals, and proximal femur. While BSIs can occur in the upper limbs and spine (especially in the lumbar spine), these are significantly less prevalent³.

Magnetic resonance imaging (MRI) is currently the gold standard for the evaluation of suspected BSIs⁴. One of the earliest and most sensitive signs of BSIs is bone marrow edema, which presents as increased signal intensity on fluid-sensitive sequences such as fat-suppressed T2-weighted or STIR (Short Tau Inversion Recovery) images, and decreased signal on T1-weighted images. This edema reflects the early inflammatory response and can be detected before any visible cortical disruption or fracture line appears³.

Despite MRI’s excellent sensitivity for detecting bone marrow changes, its ability to directly visualize bone structure is limited. Mineralized tissue, such as cortical bone, has a very short T2 relaxation time and low proton density, resulting in low signal intensity and a characteristic "signal void" on conventional MRI sequences. This limitation can hinder the detection of subtle cortical fracture lines, an essential component of BSI staging and treatment planning. As a result, computed tomography (CT) has traditionally been used for the evaluation of cortical and trabecular bone due to its high spatial resolution⁵.

However, recent advances in MRI technology are addressing this gap. Sequences such as Zero Echo Time (ZTE) and Volumetric Interpolated Breath-hold Examination (VIBE) have emerged as useful tools for bone assessment. These techniques allow MRI to depict cortical bone with a higher resolution, without the associated ionizing radiation. By providing CT-like imaging capabilities within an MRI protocol, ZTE and VIBE open the possibility of comprehensive soft tissue and bone evaluation in a single study. This article explores the technical foundations, clinical applications, strengths, and current limitations of ZTE and VIBE MRI sequences, with a particular focus on their role in BSIs assessment⁶.

IMAGING ASSESSMENT

Radiographs and bone scintigraphy have traditionally been used in the diagnosis of bone stress injuries. However, initial radiographs can appear normal in up to 90% of cases. Early periosteal reaction is infrequently observed, and visible fracture lines are even less common at the initial stages. When initial X-rays are inconclusive, bone scintigraphy may be utilized to improve sensitivity, although its specificity remains relatively low³.

In anatomical regions with a higher cortical bone proportion (e.g.: diaphysis of long bones), the earliest radiographic indicator is often the “gray cortex” sign, which refers to a subtle radiolucent area within the cortex due to osteoclastic resorption⁷. Periosteal reaction and endosteal callus formation, along with cortical thickening, tend to appear later during the reparative phase of bone healing. In higher-grade injuries, cortical discontinuity and visible fracture lines may eventually become evident⁸. Conversely, in areas where trabecular bone predominates (e.g.: metaphyseal regions), the earliest radiographic finding is a poorly defined zone of mild sclerosis. As the injury progresses, this can evolve into linear intramedullary sclerosis, reflecting bone deposition and microcallus formation. Comminution and displacement are rarely observed in stress fractures.

Given that bone marrow edema is one of the earliest detectable signs of BSIs, MRI is currently the imaging modality of choice when clinical suspicion is high. MRI offers significantly greater sensitivity and specificity than radiography in detecting stress-related bone injuries. The sensitivity of radiographs ranges from 15% to 35% in early-stage BSIs and from 30% to 70% in more advanced lesions. In contrast, MRI approaches nearly 100% sensitivity³.

Initial MRI findings include periosteal and bone marrow edema, best visualized on fluid-sensitive sequences such as T2-weighted fat-suppressed or STIR images. In more advanced cases, a low-signal fracture line may be present. As healing progresses, the formation of new periosteal and endosteal bone appears as areas of low signal intensity on all MRI sequences. Although CT is not routinely used as the first-line imaging modality in suspected BSIs, it may be valuable in cases with equivocal MRI findings, especially due to its superior resolution for evaluating cortical bone and its high specificity in confirming fracture lines⁹.

Given these imaging characteristics and the central role of MRI in the diagnosis of BSIs, incorporating MRI techniques that simulate “CT-like” bone contrast becomes essential. These sequences enhance the evaluation of cortical and trabecular bone, improving diagnostic accuracy for identifying subtle or early-stage fractures in patients with BSIs.

VIBE AND ZTE

MRI has always been the gold standard for evaluating soft tissue structures, including muscles, tendons, ligaments, and cartilage. However, its traditional limitation has been the poor visualization of bone due to the inherent properties of mineralized tissue. Cortical and trabecular bone exhibit extremely short T2 relaxation times and low proton density, rendering them nearly invisible on standard MRI sequences. On conventional T1- and T2-weighted images, bone appears as a signal void. As a result, computed tomography (CT) has been the preferred modality for assessing osseous structures because of its superior spatial resolution and ability to detect fine cortical details⁶. Nonetheless, CT involves exposure to ionizing radiation, which is particularly concerning in younger and athletic populations where repeat imaging may be necessary.

The development of specialized MRI sequences such as ZTE and VIBE has significantly expanded the utility of MRI in bone imaging. These techniques overcome the inherent signal limitations of conventional MRI by allowing the capture of rapidly decaying signals from mineralized tissues¹⁰.

ZTE employs a near-zero echo time, allowing for immediate signal capture after the radiofrequency pulse. These methods have been made possible by advances in hardware, such as radiofrequency coils, which enable high-speed switching between transmission and reception, and the introduction of higher gradient magnetic fields. This capability makes it possible to directly image cortical bone and calcifications, providing detailed anatomic visualization comparable to CT. ZTE is especially effective on 3T MRI systems, where the improved signal-to-noise ratio enhances image quality. These images can be reconstructed in three dimensions and reformatted in multiple planes, aiding in precise anatomical localization¹¹.

VIBE, on the other hand, is a T1-weighted 3D gradient-echo sequence originally designed for rapid imaging of the abdomen and breast¹². Its short acquisition time and high-resolution capabilities make it well-suited for musculoskeletal applications. In musculoskeletal imaging, VIBE provides high intrinsic contrast between bone and adjacent soft tissues such as fat and muscle. When inverted, VIBE images mimic the attenuation patterns of CT, allowing radiologists and orthopedic surgeons to interpret them in a familiar visual format. Like ZTE, VIBE supports multiplanar reformation and three-dimensional reconstruction, offering a comprehensive view of osseous and surrounding soft tissue structures in a single scan¹³.

In clinical practice, these advanced sequences have shown comparable diagnostic accuracy to CT in various scenarios. Studies by Ang et al. demonstrated that 3D T1 VIBE MRI had a 100% diagnostic accuracy for complete pars stress fractures of the lumbar spine, matching the performance of multislice CT¹⁴. Similarly, Tian et al. showed that 3D VIBE MRI had sensitivity and specificity of 93.9–97.0% for detecting bony Bankart lesions, closely approximating the 95.7–100% values observed with CT¹⁵. Stillwater et al. also found that osseous measurements obtained via 3D MRI and CT in the context of glenohumeral instability were statistically equivalent¹⁶ (Table 1).

CT-LIKE SEQUENCES AND BSIS

One of the most relevant applications of ZTE and VIBE in sports medicine is the diagnosis and monitoring of BSIs. Once MRI is the modality of choice for BSI evaluation, VIBE sequences can more clearly depict the cortical contour and can reveal subtle cortical disruptions that might be overlooked on conventional MR images. ZTE, with its high cortical bone definition, further refines the assessment by directly visualizing the integrity of the cortex 11. In grade 4b injuries (Figure 1), such as those classified by Fredericson involving the tibia, MRI provides both functional and structural insights. VIBE allows for high-resolution images that show the extent of marrow involvement, while ZTE images reveal whether there is cortical breach, which determines treatment duration and prognosis. In the spine, ZTE and VIBE have been used to confirm complete spondylolysis (Figure 2), providing a radiation-free alternative to CT, especially important in young athletes.

One of the most significant advantages of these MRI sequences is their ability to generate CT-like images without exposing patients to ionizing radiation. This is particularly relevant in sports medicine, where patients are often young and may require repeated follow-up imaging. Additionally, these sequences allow for the fusion of soft tissue and bone data in a single session, facilitating more comprehensive and efficient diagnosis. The three-dimensional reconstruction capabilities of both ZTE and VIBE sequences support precise anatomical measurements, and cross-referencing with other conventional MRI sequences¹³.

Nonetheless, there are limitations to consider. ZTE and VIBE often require specific post-processing software and expertise to generate optimal images. Their spatial resolution, while improving, remains lower than that of high-resolution CT, especially for detecting minute fractures or subtle cortical irregularities. Motion artifacts and susceptibility effects can affect image quality, particularly in uncooperative patients or areas with metallic implants. Differentiating bone from other short-T2 structures such as tendons can also present a diagnostic challenge. Furthermore, gas appears as a signal void in both sequences, potentially mimicking cortical bone and leading to misinterpretation if not carefully analyzed in conjunction with other sequences⁵.

Despite these challenges, the future of MRI bone imaging is promising. As scanner hardware, sequence optimization, and post-processing algorithms continue to evolve, the clinical utility of ZTE and VIBE will expand. Artificial intelligence integration may further streamline image processing and interpretation, increasing the adoption of these sequences in routine musculoskeletal imaging protocols.

In conclusion, ZTE and VIBE sequences represent a significant advancement in MRI technology, offering a compelling alternative to CT for bone imaging. Their growing use in sports medicine reflects a broader shift toward radiation-free, high-resolution imaging that does not compromise on diagnostic accuracy. With the ability to assess both soft tissue and bone in a single session, these sequences enhance diagnostic confidence and streamline clinical decision-making in athletes and active patients alike.

ANATOMICAL SITES AND CLASSIFICATION OF BONE STRESS INJURIES

Imaging evaluation plays a critical role in guiding treatment and estimating return-to-play timelines. The most common types of bone stress injuries are considered low-risk and include those located on the posteromedial tibia, calcaneus, third and fourth metatarsals, and the medial femoral neck.

In contrast, stress injuries occurring in areas subject to traction forces or in regions of low vascular supply are considered high-risk. These stress bone injuries are more likely to exhibit delayed healing or progress to complete fracture, requiring prolonged conservative treatment or surgical intervention most frequently. High-risk sites include the superolateral femoral neck, patella, anterior tibial cortex, medial malleolus, femoral head, dorsal navicular bone, proximal fifth metatarsal metaphysis, and the sesamoids of the hallux.

MRI findings also assist in decisions about return to play, and several MRI-based grading systems were proposed for BSIs. One of the most widely used was proposed by Fredericson et al¹⁷. This classification was developed for medial tibial stress injuries and assesses the presence and severity of periosteal, medullary, and cortical edema, as well as the presence of a fracture line. Adjacent soft tissue edema or fluid collection can also be seen, reflecting inflammation in nearby structures.

In the Fredericson system, grade 1 represents isolated periosteal edema. Grade 2 includes mild bone marrow edema visible only on T2-weighted images. Grade 3 shows more pronounced marrow edema visible on both T2 and T1-weighted images. Grade 4 represents a linear intracortical fracture. A modification by Kijowski et al. subdivides Grade 4 into 4a: intracortical signal changes without a visible fracture line; and 4b: presence of a visible fracture line¹⁸. Although originally developed for the tibia, the Fredericson system is now commonly used for stress injury classification at other anatomical sites as well.

CONCLUSION

Bone stress injuries are frequent in athletes and require early and accurate diagnosis to guide treatment and prevent progression. Advanced MRI sequences such as Zero Echo Time (ZTE) and Volumetric Interpolated Breath-hold Examination (VIBE) overcome traditional limitations in bone visualization, providing CT-like cortical detail without ionizing radiation. By combining soft tissue and osseous assessment in a single exam, these techniques improve diagnostic accuracy, support better clinical decisions, and reduce the need for additional imaging. As technology and image processing continue to evolve, ZTE and VIBE are poised to become essential tools in the evaluation and follow-up of bone stress injuries in sports medicine.

Marcelo Bordalo M.D., Ph.D.

Radiologist

Chief of Radiology

Maria Lua Sampaio Gulde M.D.

Sports Imaging Fellow

Aspetar Orthopaedic and Sports Medicine Hospital

Doha, Qatar

Contact: marcelo.bordalo@aspetar.com

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