1 Introduction

Radiology plays a crucial part in the diagnosis and management of patients. In the current times of COVID-19 radiologists play an ever increasingly important role as the keystone of diagnostic management with medical imaging, particularly with system pressures related to multifactorial issues. Diagnostic errors in radiology therefore have a considerable impact on patient care and can result in significant morbidity and in some cases mortality. The average error rate in radiology is 3–5%.^1,2

Diagnostic errors in radiology are not thought to be random occurrences, and recognizing contributing factors with a better understanding of Swiss Cheese Model can be a key factor in reducing error by developing preventive strategies.²

Review of diagnostic imaging is a specialist, comprehensive and complex process that involves pattern recognition via type 1 or automatic processes for recognizable conditions, or the more laborious and analytical type 2 processes for inconspicuous findings.¹

Specifically, there are two categories of technical error described in literature.³ The less common is a cognitive or misinterpretation error, and the more common error, which is simply failure to identify an abnormality in the first place is called a perceptual error. A plethora of causes can result in perceptual errors which include technical factors such as incorrect imaging protocol use, lack of appropriate contrast, poor lighting, non-standardized hardware or software for example reporting in different clinical setups or personal factors such as mental fatigue, inattentional blindness, satisfaction of search bias, distraction, increased speed of reporting and in some cases limited experience or/and knowledge of the radiologist.

A spectrum of techniques can be implemented to decrease the errors which include standardized imaging protocols, checklists, regular breaks, systematic review of the images and reflection and learning from mistakes.

Grayscale inversion (GSI) contrast imaging is a simple technique, which can aid in decreasing perceptual errors.

2 What is grayscale inversion (GSI) imaging?

Inversion of contrast is an easily accessible, readily available and uncomplicated tool in the arsenal of a reporting radiologist.

It has been described in literature as contrast inversion technique,⁴ grayscale inversion technique,⁵ gray scale inversion contrast imaging,^6,7 inverse digested imaging or simply as “bones black”⁸ imaging.

It is a technique of changing a positive radiographic image into a negative one.⁶ For example, on a computed tomography (CT) study, this enhancement method changes the radiodense white appearance of bone into dark gray and more importantly the gray appearing soft tissues into white. This concept can be used in other modalities including MRI, fluoroscopy and radiographs.

3 The science behind grayscale inversion imaging (GSI)

The science behind it comes from nature. Human brain is wired to identify details better in a positive polarity contrast, which is dark object against a light background. This is one of the fundamental reasons behind better perception during daylight versus night.

Studies have demonstrated that a positive polarity leads to improved perception of detail versus negative polarity (light characters on dark background), which is the cornerstone of radiological diagnosis. In fact positive polarity fares better with decreasing object size compared to negative polarity.⁹

4 How is it done

Most radiology reading softwares have an option of inverting contrast. In our institute where we use the UVWeb viewing software by General Electrics (GE), a simple click of the letter “I” on the keyboard inverts the contrast.

The technique can be applied to radiographs, CT, fluoroscopy and magnetic resonance imaging (MRI). With MRI, GSI can create images akin to CT, thus providing time and cost savings.

5 The evidence

It is by no means a novel technique. There are various studies, which have specifically looked at utilizing gray scale inversion techniques. Robinson et al⁵ reported improvement in the detectability of lung nodules on an inverted image attributed to increased nodule luminance.

Aluntesar et al. demonstrated improved mammographic evaluation and facilitation of macrocalcification detection utilizing grayscale inversion.¹⁰ Some authors have also suggested using inversion series in conjunction with convectional gray scale images to increase the diagnostic accuracy in trauma setting to diagnose rib fractures in the backdrop of minor chest trauma.¹¹

GSI has also been shown to improve not only detailing of the anatomy but delineation of pathology in temporal bones.⁷ When looking at fine measurements a study concluded that contrast inversion improved intra- and inter-observer reliabilities in measuring spinopelvic alignment when compared with a standard view, particularly in patients whose pelvic anatomical structures can't be identified clearly on the standard X-ray view.⁶

Furthermore assessment of post-operative spinal orthopaedic implants and osseous fusion has also been demonstrated to be more sensitive with gray scale inversion when compared to conventional CT.¹²

Inversion of contrast technique can also be replicated in MRI. In a study of fifty patients the authors concluded that inverted T1 VIBE sequence was excellent alternative to CT imaging which resultant in short time to diagnosis, better management and patient satisfaction as well as cost savings. This also negated the use of CT hence decreasing the radiation.¹⁴

In another study involving inversion imaging of MRI for disc pathologies, inversion imaging highlighted abnormalities better for the general radiologist as well as the surgeons.¹⁵

Whilst standard MRI sequences are limited in their detection of incomplete stress fractures,¹⁶ especially in absence of marrow edema, using GSI with 3D T1 VIBE has been shown to be 100% accurate in diagnosing complete pars fractures and is comparable to CT in detection and characterization of incomplete pars stress fractures.¹⁷ (Fig. 1). MRI also has the added advantages of detecting bone marrow edema when present and does not employ ionizing radiation, further reducing need for diagnostic and follow-up CT imaging.¹⁶

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Fig. 1 Sagittal T2 (a), T1(b) and T1VIBE inversion (c) showing pars defect of L5 (arrow).

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Other uses of GSI include VIBE MR arthrography to assess the rotator cuff and characterize osseous glenoid rim injuries. Vandevenne concluded that fast MR arthrography of the shoulder joint using VIBE sequences showed good concordance with the classically used T1-FS sequences for the appearance of the rotator cuff, in particular for large articular-sided partial thickness tears and for full thickness tears.¹⁸

Other uses of GSI in Trauma and Orthopaedics include (see Table 1).(Figs. 2–5)

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Fig. 2 Axial PDFS (a), inversion image of PDFS (b) showing tear of the anterior labrum (arrow).

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Fig. 3 Axial PDFS (a), inversion image of PDFS (b) showing Hill Sach's lesion of humeral head (arrow).

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Fig. 4 Axial T1VIBE (a), T1VIBE inversion (b) and CT showing CAM deformity of right femoral head neck (arrow).

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Fig. 5 AP (a), inversion AP(b), lateral (c) and inversion lateral (d) radiographs of the wrist showing undisplaced fracture of the scaphoid (arrow) that is better appreciated on inversion images in particular on the inversion lateral radiograph.

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Spines•Analysis of spine especially with instrumentation for loosening, fusion or fracture•Conventional hairline fractures of the spine for example of the facets

Orthopedics•Rib fractures•Hairline cortical fractures•CT arthrograms for example knee with meniscal tear•MRI arthrograms for labral tears and Hill Sach's

Given its published usefulness the pertinent question is whether it's useful for all purposes. Literature review suggests that it’s useful but when applied in conjunction with conventional images. There are studies demonstrating no benefit in additional use of inversion images for e.g. in identifying pneumothorax on chest radiographs.¹³

Some studies have even reported a negative impact of this technique, including decreased measurement accuracy of length and advised against its use.⁴

Therefore, whilst a useful adjunct in imaging it needs to be understood and applied correctly. To conclude whilst GSI is a simple tool with benefits, there are some pitfalls, which the users should be aware of as summarized below in Table 2.

Funding

No funding