A problem frequently encountered by researchers examining fruit quality disorders is trying to obtain information on the development of the disorder without destroying the fruit in the process. While certain data can be collected by simply cutting large numbers of fruit open at various times during the course of a storage trial, there are experimental advantages in being able to follow the development of disorders such as premature softening in individual fruit. The difficulty is that in most cases, symptoms are not visible on the outside of the fruit and only become apparent at the later stages of development when the fruit is in an advanced state of decay. Moreover, one can not be sure ahead of time which fruit within a sample are going to be those that are affected. Clearly what is needed is a reliable, sensitive, non-destructive technique which is capable of examining the inside of a fruit over the course of time. In the absence of advances in this area by physicists associated with the horticultural sciences we have recently started investigating the use of a technology developed by a group faced with a similar dilemma - the medical profession.

MRI - what it is and how it works

Medical practitioners have long been interested in non-invasive techniques that enable them to visualise the internal tissues of the human body. The classic examples include X-ray radiation, ultrasound, and CAT scanning. Within the last few years a further technique has entered the medical arena, Magnetic Resonance Imaging or MRI. MRI has distinct advantages over its older counterparts: it doesn't use a harmful ionising radiation; and its images provide superb definition of soft tissues like muscle and fat at high resolution.

The principles behind MRI are complex. Basically they involve transmitting radiofrequency waves into a sample held in a static magnetic field. Protons (hydrogen atoms) attached to water molecules are stimulated in this process and subsequently release energy which is converted into an electrical signal. The strength of this signal is proportional to the amount of water present. By taking a number of rapid sequential measurements of signal strength it is possible to reconstruct two- or three-dimensional images on a TV monitor, or film, which reflect the concentration and mobility of water molecules at different positions throughout the sample.

Potentially, fruit are ideal subjects to image. Their high water content ensures a strong signal. Furthermore, the water is distributed heterogeneously between different tissues so that anatomical features like the core or seeds are easily distinguished. And unlike humans, fruit are immobile and hence can be scanned for long periods of time without introducing motional artefacts. Overseas studies revealing the presence of physiological disorders within apple and pear suggested that MRI could be similarly applied to kiwifruit if an imaging system was available here. Following purchase of New Zealand's first whole-body scanner in 1991 (and approval to use it after-hours for "trivial" purposes!), we have successfully imaged kiwifruit on a number of occasions and believe that it offers a fresh perspective on the detection and development of quality disorders in this crop also. Several examples illustrate this point.

Imaging kiwifruit by MRI

The presence of large numbers of frosted fruit in May, 1992, prompted an inquiry as to whether MRI could be used to detect frost damage in trays of kiwifruit. While an entire tray was able to be imaged, the picture quality was not good enough to identify damage within a single fruit. Imaging fruit eight at a time proved more successful. Individual fruit with moderate to severe frost damage were readily identified. The dark image contrast in the locules at the calyx end of the fruit, for example, is consistent with the locality in which frost damage occurs. This was subsequently confirmed on cutting the sample open.

The medical scanner was not able, however, to assist in distinguishing between healthy fruit and those found to have lesser degrees of damage when they were opened after the imaging session. This is not the fault of the method. The magnetic field strength in medical scanners is necessarily limited by safety considerations for humans. Had the samples been able to be imaged in a research instrument containing a superconducting magnet of higher field-strength, lower levels of damage could have been detected.

MRI has also been utilised to examine the extent of injury in fruit affected by botrytis and other rots. The bright and dark regions inside a fruit containing what appeared to be a small surface rot are consistent with areas containing high and low concentrations of water, respectively. By comparison with healthy tissue in other parts of the fruit the differences in image contrast indicate the extent of the pathogen invasion.

Applications of MRI to biological samples (leaves, roots, fruit, whole plants, etc) are still in their infancy. At present, images obtained from the whole-body scanner are only being analysed qualitatively, ie, to determine whether or not damage is present, what areas of the fruit are affected, and how this changes over time. The possibilities become more exciting when instruments designed for research purposes are available rather than those dedicated to clinical applications (Fig. 1). Here one can make quantitative measurements of the water content at discrete locations within the sample, which in turn can be related to changes in other factors such as mineral and carbohydrate content, and the activity of enzymes associated with cell wall degradation in fruit during storage. A research instrument does exist at Massey University, but only for imaging samples less than 25 mm in diameter. Until larger magnets are available in New Zealand for imaging the "real thing", quantitative MRI studies involving kiwifruit will be restricted to using smaller Actinidia species as model systems.

Conclusion

Preliminary work with MRI has been sufficiently convincing for us to want to attempt more sophisticated studies that extend the capabilities of the technique beyond the qualitative assessment of an image of a fruit taken on a single occasion. One of the strengths of MRI is that the same fruit can be imaged on different occasions over a long period of time. This makes it a particularly useful research tool for examining physiological changes during ripening or storage, and for following the extent of pathogen invasion under controlled conditions.

Figure 1. Cross-sectional image slice (A) of a kiwifruit obtained on a research instrument. Even better quality images are possible by extending scanning times and increasing the number of data points (pixels) which contribute to the final image. A false colour image of a longitudinal section (B) gives quantitative information concerning the relaxation properties of water at different points throughout the sample. The relaxation property of water is a unique parameter determined by this technique, which, amongst other things, is related to the viscosity and composition of the cellular contents in the sample.

One could envisage uses for modified MRI instruments on production lines if they had real-time imaging capabilities. The technology is already developing in this direction but the multimillion dollar price tag currently associated with the technique means that you won't see these instruments appearing in the packhouse in the immediate future.

Acknowledgment

This work is being partly funded by the New Zealand Kiwifruit Marketing Board. I would also thank: Drs. Lee and Cotter and staff of the Mercy Radiology Group, Auckland, for their interest in this work and access to the scanner; and Drs MacFall and Johnson of the Radiology Dept., Duke University, North Carolina, for the opportunity to image kiwifruit using "state-of-the-art" equipment.