Advancements in preoperative imaging of pectus excavatum: a comprehensive review

Introduction

Congenital pectus deformities are a set of structural anterior chest wall abnormalities. The most common pectus deformity is pectus excavatum, which is characterized by a sunken chest wall and represents 90% of all cases (1). The disease has been associated with cardiopulmonary impairment and related symptoms such as exercise intolerance, and psychological complaints due to body image disturbances (2-4).

A sunken chest wall was first described by Bauhinus in 1594 and more detailed reports followed in the nineteenth century (1,5). These reports included anatomical descriptions of the deformation, examination of symptoms, and assessments on pathogenesis and familial predisposition (5). The first attempt to correct a pectus deformity was performed by Meyer in 1911 through rib removal surgery (6,7). Ever since, numerous techniques regarding treatment (spanning from less invasive to more invasive methods) as well as (preoperative) evaluation of pectus deformities have been recommended.

The main purposes of preoperative evaluation include diagnosis, severity classification, functional assessment, and surgical planning of chest wall deformities. Over the years, preoperative evaluation of pectus deformities has evolved from simple clinical inspection and descriptive analyses with caliper measurements of the chest to more advanced quantitative measurements and imaged documentation.

Since the discovery of X-rays in 1895 and development of the first computed tomography (CT) scanner in 1971, internal structures of the human body have been imaged extensively. These imaging methods have also led to significant evolvement in clinical evaluation of pectus excavatum, accelerating the development of numerous indices and methods to assess chest wall deformities. As such, Welch described in 1958 an index derived from a lateral chest radiograph to distinguish mild, moderate, and severe degrees of pectus excavatum from normal chest wall profiles (8,9). In 1987, the Haller index was established, describing the ratio between the maximum transverse diameter and minimum anterior-posterior distance of the chest (10).

Nowadays, several methods for preoperative assessment are routinely applied in clinical practice, each with associated advantages and disadvantages and specific applications. While preoperative imaging is mainly used to visualize the morphology of the deformation, it can also be used to quantify its effects on intrathoracic organs. Since imaging techniques are continuously advancing, new insights and opportunities are created in the clinical assessment of pectus deformities. This narrative review aims to provide current insights in conventional methods and innovative developing techniques for preoperative imaging of pectus excavatum; the most common and widely studied chest wall deformity.

Conventional preoperative imaging methods

Strategies for preoperative evaluation of pectus deformities may vary between medical centers, e.g., due to national regulations, individual preferences, a paucity of evidence and a lack of guidelines. The current paper focusses on the most commonly applied preoperative imaging methods, including thoracic CT, chest radiography, cardiac magnetic resonance imaging (MRI), echocardiography and medical photography (Table 1).

CT

CT has been a frequently applied modality for assessment of pectus excavatum (Figure 1). The calculation of pectus indices (e.g., the Haller index and correction index) and identifying cardiac compression and displacement, chest asymmetry and sternal torsion are the main applications of CT. Current developments in preoperative evaluation through CT imaging focus on automatic quantification of pectus indices, minimizing radiation dose and surgical planning (11-29).

Figure 1 Computed tomography image of a patient with pectus excavatum showing cardiac compression and displacement. Pectus indices such as the Haller index and correction index are commonly computed on CT images to quantify the deformity. CT, computed tomography.

Over the years, various indices to quantify and characterize chest wall deformities have been developed and are in general manually extracted from CT images (10,30-32). To diminish time-consuming analyses and bypass intra- and interobserver variability of manual measurements, some studies focus on (semi-)automatic evaluation of pectus deformities. Between 2008 and 2010, Kim et al. proposed four indices (the eccentricity index, flatness index, circularity index, and rotation index) that quantify the depression and asymmetry of pectus excavatum and designed a fully automatic pipeline to calculate these indices using several image processing techniques, such as a segmentation algorithm and a curve fitting (active contour) model (11,12). A few years later, they extended their method and proposed two other indices (the depression ratio and asymmetry ratio) to provide a linear output for the severity of symmetric, asymmetric and complex chest wall deformities (13).

In contrast to an index-based evaluation of pectus excavatum severity, Lai et al. [2020] proposed a non-index-based approach, aiming to represent the full chest shape and exploit the volumetric information in chest CT images (14). They developed an automatic classification model based on deep learning that differentiates each CT slice as normal, mild, or severe pectus excavatum. Subsequently, a majority rule-based voting strategy is applied across the entire CT volume to draw a final diagnostic conclusion on the pectus severity.

Although these algorithms have been proposed in the past 15 years to automatically extract novel indices that evaluate pectus severity from CT images, none aimed to develop automated methods to calculate conventional and clinically used pectus indices such as the Haller index and correction index. Only recently, in 2022, Silva et al. applied a UNet++ architecture that was evaluated on a database of 269 CT scans to automatically calculate the Haller index, correction index and asymmetry index (15).

The greatest concern regarding the use of a CT scan for evaluating pectus deformities, particularly in pediatric and adolescent patients, involves the relatively high dose of ionizing radiation. In an effort to reduce the CT-associated radiation dose for pectus excavatum assessment, a so-called very low-dose protocol has been suggested, consisting of five to seven slices centered on the area of maximal sternal depression (16). This method reduces the radiation dose from 1–2 mSv in full chest CT to approximately 0.35 mSv, while still allowing the calculation of the Haller index and displaying local anatomy of interest.

Two other recent studies focused on the development of a CT imaging protocol to minimize radiation exposure. Gosztyla et al. [2022] proposed a mini-thoracic CT for the calculation of the Haller index (17). This mini-CT involves the placement of a radio-opaque marker at the point of most severe depression, after which a column of 3 cm above and 3 cm below the marker is scanned using 5 mm slices. Compared to a general low dose chest-CT of 10 mm slices from the sternal inlet to the costal margin, the mini-CT reduced the effective dose (mSv) by 63% (0.88 versus 0.32 mSv). The effective dose can be brought down even further to less than 0.15 mSv by implementing an ultralow-dose CT protocol (18,19). Gallo-Bernal et al. [2022] compared an ultralow-dose CT protocol (70–80 kV; fixed tube current of 50 mAs; slice thickness 2.5 mm) to a standard-dose CT protocol (100–120 kV; variable tube current, smart mA; slice thickness 2.5 mm) by evaluating the effect on image quality as well as the capability to detect non-osseous thoracic pathology (18). A scanning range from the thoracic inlet to the level of the adrenal glands was used and scans were reconstructed using adaptive statistical iterative reconstruction, which supports preservation of diagnostic image quality in low-dose CT acquisition. The signal-to-noise and contrast-to-noise ratios were significantly higher in standard CT images and 3 out of 104 ultralow-dose scans were considered insufficient to diagnose thoracic pathology beyond pectus excavatum.

Besides using CT for pectus evaluation, CT is considered a well-suited imaging modality for surgical planning of pectus excavatum repair. Surgeons may devise surgical strategies based upon CT-derived three-dimensional (3D) reconstruction of the thoracic wall (20). Other emerging techniques to aid in the decision process are 3D printing (additive manufacturing) of thoracic models and bar templates, or virtual simulations of the Nuss procedure (21-29). Preoperative planning using CT scans allows to create personalized pectus bars preoperatively for more effective corrections of the deformity and to eliminate intraoperative revisions of the bar shape. However, whether these technologies improve surgical accuracy and reduce surgical time and costs compared to conventional techniques remains to be elucidated.

Chest radiography

Chest radiography is an alternative imaging method to visualize and assess the degree of chest wall deformity (Figure 2). Aside from a depression of the sternum, characteristic findings of pectus excavatum on chest radiographs include a leftward displacement of the cardiac silhouette, obscured right cardiac border, narrowing of the mediastinum, and slanted anterior ribs (33). Although chest radiography is unsuitable to evaluate chest asymmetry or sternal torsion, various pectus indices to assess the deformity can be determined from plain radiographs (e.g., Welch index, vertebral index, body manubrium xyphoid index, Haller index, correction index) (8,10,30,32,34,35).

Figure 2 Chest radiographs of a patient with pectus excavatum. (A) Frontal projection displaying an obscured right cardiac border and slanted anterior ribs. (B) Lateral projection showing the posterior position of the sternum.

Both the Haller index and the correction index are originally CT-derived indices, but it has been shown that these indices can be measured reliably and accurately based on chest radiographic images (36-39). By replacing a full chest CT scan with a two-way chest radiograph, the estimated reduction in radiation exposure is in the order of 100-fold (1.1–4.1 versus 0.0198 mSv), while associated costs are decreased by approximately 90% (37,38). To further refine the measurement accuracy of the Haller index and correction index on radiographic images, potential confounders were investigated by McHugh et al. [2016]. The accuracy of both the Haller index and correction index measurements were not influenced by image quality, scoliosis and an eccentric type of pectus asymmetry. Nevertheless, caution is advocated when using chest radiography alone in cases of an unbalanced type of asymmetry (i.e., a disproportionate depression on one side of the chest compared to the contralateral side, with a difference in angles of >10 degrees), since this method was found to decrease measurement accuracy of the correction index when compared to CT measurements (40).

In addition to preoperative application of two-view chest radiography, single anterior-posterior radiographs of the chest are often obtained directly postoperatively to assess the position of implanted pectus bars and to detect complications such as a pneumothorax.

MRI

Traditionally, preoperative imaging in patients with pectus excavatum has been centered around radiographic imaging. However, over the past two decades, MRI has emerged as an alternative imaging method in this context (Figure 3). Similar to CT, an MRI allows for the (semi-automatic) calculation of traditional pectus indices (41), with the added advantage of imaging without exposure to ionizing radiation and the possibility to evaluate dynamic cardiac function (42-49).

Figure 3 Magnetic resonance image of a patient with pectus excavatum showing cardiac compression.

Conventional methods to evaluate myocardial function on MRI are through the assessment of wall motion abnormalities or ejection fraction. However, imaging of ventricular myocardial strain (i.e., fractional deformation of the myocardial wall throughout the cardiac cycle) has been shown to detect myocardial dysfunction at an earlier stage compared to conventional measures (50-52).

In case of pectus excavatum, assessment of cardiac function is mainly focused on the right ventricle since the associated sternal depression usually deforms the right ventricular wall (53). To improve cardiac MRI assessment in patients with pectus excavatum, Rodriguez-Granillo et al. [2022] evaluated systolic function of both the right and left ventricles using longitudinal cardiac strain and conventional ejection fraction analyses (50). The study demonstrated that right ventricular systolic dysfunction can be identified using conventional analyses, while subtle left ventricular systolic dysfunction could only be detected using longitudinal strain analysis and was overlooked by conventional assessment (50). Though, it must be remembered that these results are based on analyses of cardiac function of patients at rest and in supine position. Exercise cardiac MRI is a developing technique which may offer a more comprehensive assessment of cardiac function during exercise (54). Still, this technique requires further advancements and its clinical utility in the preoperative assessment of pectus excavatum has yet to be evaluated.

Evaluating chest wall deformities is generally based on static axial cross-sectional images of the chest because standard imaging techniques require breath-holding procedures. Although it has been demonstrated that thoracic indices, such as the Haller index and correction index, vary during breathing cycle, the phase at which the indices should be determined has not been standardized. The added value of MRI compared to other imaging methods is the possibility for real-time imaging. In a recent study (55), dynamics of the chest wall in 56 patients with pectus excavatum have been observed using real-time MRI. The results showed that between maximum inspiration and maximum expiration, the Haller index passed through the clinical threshold of 3.25 in only 4 subjects (7%), while the correction index passed its threshold of 2% in 15 subjects (27%). Although measurement variance was not addressed in this study, dynamic real-time MRI in the assessment of pectus excavatum may provide clear visualization and a better understanding of the compression by the sternum on the heart and lungs and thus helps in clinical decision-making. In case of conventional static imaging of the chest wall, the authors recommend calculating thoracic indices in the end-expiratory phase of quiet breathing, since the respective indices are not significantly dependent on the degree of expiration and the intraoperative situation is resembled more closely by the end-expiratory breathing phase (55).

Echocardiography

Echocardiographic imaging (see Figure 4) may be useful for cardiac evaluation, as it can reveal structural and functional changes of the right ventricle (e.g., compression of the right atrium and ventricle; systolic and diastolic disfunction) and heart valves (e.g., tricuspid- and mitral valve prolapse) as well as the presence of (congenital) anomalies (e.g., septal defects, aortic root dilation and pericardial effusion) (56,57). A dilated aortic root and mitral valve prolapse can for example indicate Marfan’s syndrome. Transthoracic imaging is often challenging in patients with pectus excavatum due to the deformation of the thoracic and cardiac geometry. In some cases a subcostal view may provide adequate images, but transesophageal imaging can largely overcome the limited view during imaging (57,58). However, this technique is practically more inconvenient and often requires sedation of the patient.

Figure 4 Echocardiographic examination of a patient with pectus excavatum, visualizing compression of the right atrium and ventricle, with minor diastolic dysfunction.

Although echocardiography is performed to identify potential cardiac abnormalities, echocardiographic approaches are of limited accuracy in evaluating cardiac function in patients with pectus excavatum. Qualitative assessment (e.g., tricuspid annular plane systolic excursion) is based on geometrical assumptions which not necessarily hold true in this anatomically distinct population (59). These limitations may explain why there is only limited data on the application of echography in the preoperative assessment of pectus excavatum. Nevertheless, a study from 2020 based on a relatively large cohort (269 patients) analyzed findings from stress echocardiography and explored the relationship with the severity of chest wall malformation based on the Haller index and correction index. Although the mechanisms involved are not fully elucidated, right ventricular diastolic as well as systolic function appeared to worsen during exercise and both were demonstrated to be associated with the Haller index and correction index (60).

Medical photography

Medical photography is a relatively simple, inexpensive, and non-invasive method to image and document anatomical malformations of the chest (Figure 5). Prior to surgical correction of pectus excavatum, it is useful to identify the presence of asymmetry, costal flaring and sternal torsion and the extent of depression (61). Medical photography of pectus excavatum is often combined with a caliper measurement to provide a quantitative measure of the excavation depth (62,63). A reliable protocol for photographic documentation of pectus excavatum consists of three types of recordings: a series of five photographs from different angles (left lateral, left oblique, anterior, right oblique, right lateral) with additional lighting to accentuate the shape of the deformity, a photograph of a caliper measuring the depth of the excavation, and a 3D grid-projection photograph (i.e., raster stereography) (62). By comparing preoperative and postoperative photographs, the result of the surgical correction can be properly assessed and monitored over time.

Figure 5 Medical photography of pectus excavatum with (A) additional lighting to accentuate the shape of the deformity, (B) raster stereography in which the distortion of the grid lines visualizes the depth of the deformity and (C) a caliper measurement to quantify the deformation.

Emerging preoperative imaging: 3D optical surface imaging

An emerging imaging technique to preoperatively evaluate pectus deformities is 3D optical surface imaging, which maps the external contour (i.e., skin surface) of the thoracic wall (Figure 6). This optical scanning technique offers the acquisition of images as fast as within a few seconds, without ionizing radiation. Such scanning devices can be based on several different technologies to capture and reconstruct image data, each having distinct advantages and limitations with a broad range of costs. Most scanning devices used for chest wall imaging are based on structured light projection technology in which the distortion of a pattern of light due to the thoracic geometry allows for the measurement of protrusions and depression in the examined surface. A 3D surface scan can be represented by point clouds or a polygonal mesh that can be analyzed manually or automatically to produce quantitative measurements. Several studies have investigated the application of optical scanning in the preoperative evaluation of pectus deformities and demonstrated that a modified pectus index based on the skin surface (i.e., an external-derived Haller or correction index) correlates with the conventional Haller and correction index (64-72). In addition, cut-off values for externally derived indices to help determine surgical candidacy have been established. The optimal cut-off value for the external Haller index was calculated at ≥1.83 (corresponding to the conventional Haller index ≥3.25) and the external correction index at ≥15.2% (corresponding to the conventional correction index ≥28.0%) (73). Along with the calculation of equivalents to conventional pectus indices, optical imaging may be used to quantify other morphological features of pectus excavatum (74) and can predict the presence of cardiac compression without the use of CT imaging (75). Combined with automatic algorithms and predictive models, 3D surface scans may aid in preoperative decision-making and visualization of expected aesthetic outcomes after surgery (76).

Figure 6 Three-dimensional optical surface image of a patient with symmetric pectus excavatum predominantly affecting the inferior region of the sternum.

Nevertheless, these developments are still in their infancy and require further evaluation and validation. Prior to widespread use of 3D optical imaging, additional issues should be addressed regarding accuracy of measurements in patients with breasts or excess body fat, standardization of imaging systems and analyses, cost and availability of 3D imaging systems in medical centers.

Conclusions

Preoperative imaging of pectus excavatum comprises a variety of imaging modalities which have been evolved extensively over the years. Current advancements in preoperative imaging focus on radiation reduction, automatic quantification of pectus indices, and surgical planning. MRI offers valuable advantages, as it permits the (semi-automatic) calculation of traditional pectus indices, as well as the assessment of cardiac function within a single, radiation-free examination. Furthermore, the capacity for real-time imaging can provide insight into the dynamic nature of chest wall deformities. Chest radiography and echocardiography are alternative procedures, but future technological developments are mainly focused on other imaging techniques. The latest emerging imaging technique is three-dimensional optical surface imaging, which may be applied as a non-invasive method to calculate equivalents of conventional pectus indices and other features of pectus excavatum. As a currently developing technology, 3D imaging holds promising potential for future advancements in the field.

Acknowledgments

Funding: None.

Footnote

Provenance and Peer Review: This article was commissioned by the editorial office, Journal of Thoracic Disease, for the series “Minimally Invasive Treatment of Pectus Deformities”. The article has undergone external peer review.

Peer Review File: Available at https://jtd.amegroups.com/article/view/10.21037/jtd-23-662/prf

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jtd.amegroups.com/article/view/10.21037/jtd-23-662/coif). The series “Minimally Invasive Treatment of Pectus Deformities” was commissioned by the editorial office without any funding or sponsorship. E.R.d.L. and J.H.T.D. served as unpaid Guest Editors of the series. The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All clinical procedures described in this study were performed in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Helsinki Declaration (as revised in 2013). Written informed consent was obtained from the patients for the publication of this article and accompanying images.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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