Abstract
The evolution of dedicated magnetic resonance imaging (MRI) musculoskeletal equipment allows new sequences and better images of the nail unit. The use of MRI has modified the imaging strategies used in treating inflammatory arthritis. In the case of psoriatic arthritis (PsA), the MRI study of the nail unit identifies nail involvement, which appears as an initial lesion for the induction of distal phalanx damage and consequently of distal interphalangeal joint arthritis. All patients with psoriasis, even in the absence of a clinically evident onychopathy, show characteristic MRI changes in the nail. This evidence could have a practical diagnostic value, because MRI study of the nail could document diagnosis in patients with undifferentiated spondyloarthropathies who have a barely evident psoriasis. We discuss the advantages and problems related to the use of low-field and high-field MRI in the study of the nail unit of patients with PsA.
The correlation between the involvement of nails and distal interphalangeal (DIP) joints in psoriatic arthritis (PsA) is well proven1,2,3,4. In the past, radiographic study of the distal phalanx in patients with PsA was used to study the relationship between nail and bone changes. Although DIP joint changes correlated with the duration of onychopathy, no statistical difference was found in the distribution of DIP joint arthritis in PsA patients with or without nail disease.
In the hypothesis of a pathogenetic link between bone changes of the distal phalanx and onychopathy of the adjacent nail, we used magnetic resonance imaging (MRI) to study the nail profile, the distal phalanx, and DIP joints of patients with PsA5 (Figure 1B, IC). We found that nail involvement was present in almost all cases studied, even in those without clinically evident onychopathy. Nail thickening with or without surface irregularity was the most common finding, and MRI nail changes were more marked in patients who had an increased Nail Psoriasis Severity Index, a score for clinical changes of the matrix and of the nail bed6. In addition, we demonstrated the constant overlap between the presence of nail alteration and distal phalanx involvement. The involvement of the DIP joint was present instead in a smaller percentage of patients always showing distal phalanx involvement. This point suggested a primary involvement of entheses linking nail to distal phalanx and supported the idea that the involvement of the DIP joint could be secondary to that of the distal phalanx4,5.
A. High-field magnetic resonance imaging (MRI) of the distal phalanx, fast-field echo (FFE) T1 3-D axial image, executed after applying petroleum jelly (asterisks) on the nail to identify its outer margin, in a control subject in which the axial plane allows exhaustive study of the nail, which appears as a physiologic low signal of regular shape and thickness (arrows). B. High-field MRI of the distal phalanx, FFE T1 3-D axial image, in a patient with psoriatic arthritis without a clinically evident onychopathy. Asterisks indicate outer margin of nail. The nail is asymmetrically thickened (arrows). C. Low-field MRI of the distal phalanx, turbo spin echo T1 axial, in a patient with PsA with a clinically evident onychopathy. Asterisks indicate outer margin of nail. The nail is thickened with an irregular outer margin (arrows).
We obtained similar results also in fingers studied by dedicated low-field MRI apparatus7 (Figure 1C); we have also tested a prototypal finger coil that gave us good results and images similar to or better than those obtained on commercial instruments8 (Figure 2A). In this kind of coil we obtained, in less time, better images with a higher resolution when compared with the standard wrist coil.
A. Dedicated prototype finger coil. Low-field magnetic resonance imaging of the distal phalanx, turbo spin echo T1 axial (B), coronal (C), and sagittal (D) executed after applying petroleum jelly on the nail to identify its outer margin, all in same patient with psoriatic arthritis.
The new generation of dedicated low-field MRI equipment, also with standard coils, allows the study of the nail with 3-D acquisition and complex sequences, giving more information about bone and soft tissue.
USEFULNESS IN CLINICAL PRACTICE OF MRI STUDY OF THE NAIL UNITS OF PATIENTS WITH ARTHRITIS
As we demonstrated, the use of MRI has modified the imaging strategies of ungual and subungual diseases9,10. The traditional radiographic plain film study is limited to the evaluation of bone structures. The MRI allows the evaluation of bone, soft tissue, and if adequately performed, the nail11,12.
In the case of PsA, the MRI study of the nail unit revealed that nail involvement is the main lesion for the development of the distal phalanx damage and consequently of DIP joint arthritis. We also demonstrated that all patients with psoriasis, even in the absence of clinically evident onychopathy, show characteristic MRI nail changes. So MRI study of the nail could be used in patients belonging to the undifferentiated subset of spondyloarthropathies who have a barely evident psoriasis; the presence of typical nail psoriasis changes could permit a more precise classification of the cutaneous preclinical stage13.
Moreover, as we have demonstrated, MRI of the nail could play a role in diagnostic differentiation in the case of patients with other inflammatory conditions such as rheumatoid arthritis (RA). In patients with RA, MRI has revealed the involvement of the DIP joint but it does not show alterations of the nail14. In addition, MRI could allow differential diagnosis between PsA and nodal osteoarthritis in patients with associated skin psoriasis.
MAGNETIC RESONANCE IMAGING
As reported, MRI of the finger, in the absence of ionizing radiation, is useful for the evaluation of patients who have borderline psoriasis.
The nail margin can be evaluated with MRI by putting petroleum jelly on its outer surface to outline the nail profile and to allow easy evaluation of the phalanx bone alterations and tendon involvement. The sensitivity of MRI in revealing bone erosion, bone edema, and soft tissue inflammation is higher than with plain radiograph studies9,10,11,12. The association with intravenous contrast medium administration, for example, is widely used to demonstrate synovial inflammation in RA and to increase the sensitivity in identifying bone alterations, before their morphological evidence15.
When we started to perform nail studies, only high-field MRI (1.5 T superconductive magnet) was available. Study of the nail on high-field MRI was considered expensive and difficult to perform because it reduces the time that the machine is available to study other more severe and life-threatening pathologies such as those in the brain or abdomen, but also for vascular examinations, functional studies, spectroscopy, and so on. Moreover, to obtain high imaging quality in closed magnets, patients must keep an uncomfortable position with the hand above the head – a posture that can be difficult for patients with claustrophobia to assume and to keep, as well as for patients with arthritis.
Therefore, we started to perform similar studies on low-field dedicated musculoskeletal MRI equipment. These machines are cheaper, cost less to install, take up less space, and are easier to use. The absence of a superconductive magnet avoids helium usage, reducing maintenance costs. The open structures allow easy access for the patients, avoiding claustrophobia and reducing anxiety; patients do not need to assume an uncomfortable position. During examination, they simply lie with the arms alongside the body. The anatomical region of interest, in our case the finger, is in the very center of the coil in the magnet. The patient is relaxed and movement artifacts are drastically reduced. Another point to be considered in low-field MRI is the reduction of safety risks mainly linked to the magnetic field strength (e.g., interaction with ferromagnetic or iron objects inside the patients).
The major drawback is a lower signal-to-noise ratio with more imaging noise: this means longer acquisition time. Another drawback was the lack of complex fat-suppressed sequences such as spoiled gradient echo or spectral saturation by inversion recovery, and the impossibility of performing 3-D complex sequences.
Our first studies showed a low sensitivity for bone marrow edema because of the small anatomical region examined. With regard to image resolution, the actual generation of open devices permits diagnostic images comparable (for the articular studies) to those of the high-field MRI; this is also confirmed in our study13, in which results obtained on high-field MRI were comparable to those obtained on low-field dedicated machines.
DEDICATED MRI
We now have the opportunity to perform the examination with dedicated coils, to obtain a higher signal-to-noise ratio with a smaller field of view (FOV). The tested prototype of a dedicated finger coil allowed us to obtain better images in less time with a higher resolution compared with the standard wrist coil8 in the same machine. The advantage of this coil, in addition to the resolution, is that it is easy to place the finger in the small hole at the center of the coil. This allows the region of interest to be easily centered in the magnetic field, in turn allowing a smaller FOV. In this way the signal is significantly increased and pixel size can be reduced, without image degradation, giving a higher resolution with a shorter time execution (Figures 2B–D).
Available dedicated MRI allows us to obtain better images than with the equipment of 10 years ago. The magnetic field is more or less the same, but new hardware and excellent new sequences now available allow images of higher quality and resolution. These kinds of sequences were intended for the study of the spine and for the evaluation of knee cartilage, but are also ideal for the study of the nail, allowing 3-D steady state high-contrast images; this means high spatial resolution, good contrast, and the possibility of multiplanar reconstruction on a single 7-min acquisition (Figure 3).
Low-field new generation magnetic resonance imaging of the distal phalanx, axial 3-D gradient echo T1 (A), executed after applying petroleum jelly on the nail to identify its outer margin in a patient with psoriatic arthritis, with standard wrist coil. Three-D images allow a high resolution and a bigger field of view to evaluate bone and nail (B).
The 2-D base sequences have also been improved, and with the same contrast and same resolution, compared with high-field images. Fat suppression is also enhanced, and complex images subtracted using the Dixon method can be used to identify bone or soft tissue edema (Figures 4 and 5).
Low-field new generation magnetic resonance imaging of the distal phalanx, short-tau inversion recovery (STIR) sagittal, standard wrist coil. In the STIR image, thumb’s normal bone and subcutaneous tissue appear dark due to suppression of fat signal.
Low-field new generation magnetic resonance imaging of the distal phalanx, short-tau inversion recovery (STIR) coronal, standard wrist coil. Index, middle, and ring fingers in patient with psoriatic arthritis. In the STIR image, bone and soft tissue edema is clearly recognizable by high-intensity areas in the dark healthy structures.
MRI, either with high-field or low-field apparatus, supports the diagnoses of rheumatologists and opens new perspectives in evaluation of patients with rheumatic conditions13,14,15,16. In particular it could be very helpful in cases of early or uncertain diagnosis of PsA.
